Hydrogen diffusion barrier for hybrid semiconductor growth

ABSTRACT

Semiconductor devices and methods of fabricating semiconductor devices having a dilute nitride active layer and at least one semiconductor material overlying the dilute nitride active layer are disclosed. Hybrid epitaxial growth and the use of hydrogen diffusion barrier layers to minimize hydrogen diffusion into the dilute nitride active layer are used to fabricate high-efficiency multijunction solar cells and photonic devices. Hydrogen diffusion barriers can be formed through the use of layer thickness, composition, doping and/or strain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of Ser. No. 16/535,874,filed on Aug. 8, 2019, now U.S. Publication No.: US 2020/0052137-A1,which claims the benefit under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication No. 62/716,814 filed on Aug. 9, 2018, which are incorporatedby reference in their entirety.

FIELD

The present invention relates to semiconductor devices and to methods offabricating semiconductor devices having a dilute nitride active layerand at least one semiconductor material overlying the dilute nitrideactive layer. Particularly, the present invention relates to hybridepitaxial growth of high-efficiency multijunction solar cells and dilutenitride photonic devices.

BACKGROUND

Epitaxial growth of III-V materials is a cornerstone technology for thewireless, optical and photovoltaic industries. Structures such aspseudomorphic high electron mobility transistors (PHEMTs),heterojunction bipolar transistors (HBTs), vertical-cavitysurface-emitting lasers (VCSELs) and multijunction solar cells requirethe purity and crystalline quality that only epitaxial growth canprovide. Two technologies used to fabricate multijunction solar cellsare molecular beam epitaxy (MBE) and metal-organic chemical vapordeposition (MOCVD, or metal-organic chemical vapor deposition, MOVPE, ororganometallic vapor phase epitaxy, OMVPE).

Dilute nitrides are a class of III-V semiconductor alloy materials(alloys having one or more elements from Group III in the periodic tablealong with one or more elements from Group V in the periodic table) withsmall fractions (less than about 7 atomic percent or 5 atomic percent,for example) of nitrogen). Dilute nitrides are of interest since theyhave a lattice constant that can be varied to be substantially matchedto a broad range of substrates, including GaAs and germanium, and/orother semiconductor layers such as subcells for photovoltaic cellsformed from materials other than dilute nitrides. The lattice constantcan be controlled by the relative fractions of the different group IIIAand group VA elements. Thus, by tailoring the compositions (i.e., theelements and quantities) of a dilute nitride material, a wide range oflattice constants and band gaps may be obtained. Further, high qualitymaterial may be obtained by optimizing the composition around a specificlattice constant and band gap, while limiting the total antimony contentto no more than 20 percent of the Group V lattice sites, such as to nomore than 3 percent of the Group V lattice sites, or to no more than 1percent of the Group V lattice sites.

Although metamorphic structures for III-V multijunction photovoltaiccells can be used, lattice-matched dilute nitride structures arepreferred due to band gap tunability and lattice constant matching,making dilute nitrides ideal for integration into multijunctionphotovoltaic cells with substantial efficiency improvements. Dilutenitrides have proven performance reliability and require lesssemiconductor material in manufacturing. The high efficiencies of dilutenitride photovoltaic cells make them attractive for terrestrialconcentrating photovoltaic systems and for photovoltaic systems designedto operate in space. Dilute nitrides are also of interest for photonicdevices such as photodetectors and semiconductor lasers such as VCSELs.Significantly, thermal treatment is an essential and unique step in thefabrication of dilute nitride devices, which is not required forconventional semiconductors. A thermal load is required to amelioratestructure defects within the dilute nitride material.

Although MOCVD is a preferred technology in solar cell commercialproduction, plasma-assisted MBE is used for growing dilute nitridematerials having a band gap of about 1 eV. It is difficult toincorporate a sufficient mole fraction of nitrogen by MOCVD into thelattice of epilayers. Plasma-assisted MBE offers superior dilute nitridecomposition control and material quality, in part because the process isable to produce more nitrogen radicals, which increases nitrogenincorporation into the semiconductor layers to reduce the band gapwithin a range from about 0.7 eV to 1.2 eV. Other junctions in amultijunction solar cell (e.g., (Al)GaAs, (Al)(In)GaP)) can be grown byeither MBE or MOCVD with comparable performance and quality.

MBE growth occurs on a heated substrate in an ultra-high vacuum (UHV)environment (with a base pressure ˜1E-9 Torr) typically using elementalsources without a carrier gas. The UHV environment ensures materialpurity. Layered structures are achieved by shuttering. It can bechallenging to adapt MBE to commercial production.

MOCVD growth occurs on a heated substrate in a totally differentpressure regime than MBE (typically 15 Torr to 750 Torr). Unlike MBE,MOCVD uses complex compound sources, namely metal-organic sources (e.g.,tri-methyl Ga, In, Al, etc.), hydrides (e.g., AsH₃, etc.), and other gassources (e.g., disilane). In MOCVD, the reactants flow across thesubstrate where they react with the surface resulting in epitaxialgrowth. In contrast to MBE, MOCVD requires the use of a carrier gas(typically hydrogen) to transport reactants across the substratesurface. Layered structures are achieved by valve actuation fordiffering injection ports of a gas manifold. Maintenance of the MOCVDapparatus is much more frequent than for an MBE apparatus but less timeconsuming. Therefore, MOCVD is able to recover more quickly fromequipment failures or reconfiguration. MBE, on the other hand, involveslonger maintenance periods and has setup variability limitations. MOCVDis the preferred technology in commercial production due to loweroperational costs.

Hydrogen gas is often used as a carrier for arsenide and phosphidegrowth, and therefore semiconductor materials grown by MOCVD can beunintentionally doped with hydrogen. During epitaxy, hydrogen gas canarise from (1) the hydrogen gas carrier itself, and (2) through crackingof arsine or phosphine at the semiconductor surface, during whichcovalent bonds are broken and hydrogen is released. In contrast, MBEepitaxy uses solid or plasma sources without carrier gases, whicheliminates complications resulting from the presence of hydrogen in thereactor. Once epiwafers are transferred from a low-hydrogen (MBE)environment to a hydrogen-rich (MOCVD) environment, hydrogen gas candiffuse into MBE-grown semiconductor layers and causepassivation-compensation and/or introduce isolated donor or defects inthese layers, for example, complex defects of nitrogen and hydrogen,such as N—H and N—H-VGa (where VGa is associated with galliumvacancies). Additionally, MBE growth of semiconductor materials on anunderlying MOCVD-grown semiconductor structure may cause hydrogen withinthe underlying MOCVD layer to diffuse into the MBE-grown materials.Unintentional hydrogen doping will contaminate and degrade a dilutenitride active layer purposely grown at slow rates in the ultra-highvacuum MBE. Each epitaxial growth technique has its specific merits inspecific device applications. For this reason, new and improvedMOCVD/MBE hybrid epitaxial growth techniques and structures are requiredto harness the benefits and mitigate the down-sides of both techniques.

Successful implementation of MOCVD/MBE hybrid epitaxy requires properprotection of grown intermediate epitaxial layers so that the topsurfaces of such layers remain pristine and “epi-ready” for overgrowth.Oxidation or contamination of the interface layers must be prevented tomake hybrid growth viable. The layers should also reduce or preventdiffusion of hydrogen from MOCVD growth into underlying and/or oroverlying dilute nitride active layers and should also be able towithstand thermal treatments used in dilute nitride epitaxialprocessing. Use of sacrificial layers as protective or cap layers to beetched away prior to subsequent growth steps is inefficient, especiallyin high-volume production.

SUMMARY

This disclosure describes the design of an epitaxial structure andgrowth schedule that minimizes surface contamination and defects asepitaxial growth is interrupted in one reactor (MBE or MOCVD) and thenresumed in a different reactor. The structures and processes alsomitigate the effects of diffusion of hydrogen from MOCVD growth intounderlying and/or or overlying MBE-grown dilute nitride active layers.

Dilute nitride electronic devices described in this disclosure resultfrom the successful implementation of the MOCVD/MBE hybrid growthmethod. High efficiency devices result from specific epitaxial structuredesign (e.g., layer thicknesses and doping profiles), growth conditions(e.g., temperatures during growth and idle times, as well as growthrates), and deliberate partial or full annealing of dilute nitrideactive layers during the hybrid fabrication process with minimizeddegradation to other junctions. The MOCVD/MBE hybrid method can also beapplied to non-solar opto-electronic/photonic devices that incorporateat least one dilute nitride active layer, such as lasers,vertical-cavity surface-emitting lasers (VCSELs), detectors, and powerconverters.

According to embodiments of the present invention, semiconductor devicescomprise a hydrogen diffusion barrier region overlying and/or a dilutenitride active layer, wherein the hydrogen diffusion barrier comprises adoped layer.

According to some embodiments of the present invention, semiconductordevices comprise a hydrogen diffusion barrier region overlying a dilutenitride active layer, wherein the hydrogen diffusion barrier comprises astrained layer.

According to the present invention, methods of fabricating asemiconductor device comprise a dilute nitride active layer, comprisingproviding a substrate; growing a dilute nitride active layer overlyingthe substrate using molecular beam epitaxy; growing a hydrogen diffusionbarrier region overlying the dilute nitride active layer using molecularbeam epitaxy; applying a first thermal treatment to the substrate, thedilute nitride active layer, and the hydrogen diffusion barrier region;growing one or more semiconductor layers overlying the annealed hydrogendiffusion barrier region using metal-organic chemical vapor deposition;and applying a second thermal treatment to the substrate, the dilutenitride active layer, the hydrogen diffusion barrier region, and the oneor more semiconductor layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art will understand that the drawings describedherein are for illustration purposes only. The drawings are not intendedto limit the scope of the present disclosure.

FIGS. 1A-1C show examples of junction compositions for 3J (3-junction),4J (4-junction) and 5J (5-junction) multijunction photovoltaic cells.

FIGS. 2A-2B show cross-sections of embodiments of a multijunction solarcell comprising a dilute nitride active layer and a hydrogen diffusionbarrier region overlying the dilute nitride active layer.

FIGS. 3A-3B show cross-sections of other embodiments of a multijunctionsolar cell comprising a dilute nitride active layer and a hydrogendiffusion barrier region overlying the dilute nitride active layer.

FIG. 4 shows a cross-section of an embodiment of a photonic devicecomprising a dilute nitride active layer and a hydrogen diffusionbarrier region overlying the dilute nitride active layer.

FIG. 5A-5D show summaries of the composition and function of certainlayers/regions that may be present in embodiments of 4J multijunctionsolar cells comprising AlInGaP/(Al)(In)GaAs/GaInNAsSb/Ge, according tothe present disclosure.

FIG. 6A-6E show process flow steps according to methods provided by thepresent disclosure.

FIG. 7 shows a cross section of a strained layer superlattice hydrogendiffusion barrier.

FIG. 8 shows a cross-section of an embodiment of a photonic devicecomprising a dilute nitride active layer, a hydrogen diffusion barrierregion underlying the dilute nitride active layer and a hydrogendiffusion barrier region overlying the dilute nitride active layer.

FIG. 9 shows a cross section of a vertical-cavity surface-emitting lasercomprising a dilute nitride active layer and a hydrogen diffusionbarrier region overlying the dilute nitride active layer.

DETAILED DESCRIPTION

The devices and methods of the present disclosure facilitate themanufacturing of high quality electronic and optoelectronic devices thatresult from successful implementation of MBE/MOCVD hybrid epitaxy. Thedevices and methods disclosed include details that pertain to dilutenitride multijunction solar cells, and photonic devices includingphotodetectors and lasers.

The composition of a dilute nitride can be modified to achieve a widerange of lattice constants and band gaps. Examples of suitable dilutenitrides include GaNAs, GaInNAs, GaInNAsSb, GaInNAsBi, GaInNAsSbBi,GaNAsSb, GaNAsBi and GaNAsSbBi. High quality dilute nitrides can beobtained by tailoring the quantities of each element around a specificlattice constant and band gap, while limiting the total Sb content to nomore than 20 percent of the Group V lattice sites, such as to no morethan 3 percent of the Group V lattice sites, or to no more than 1percent of the Group V lattice sites. Antimony, Sb, is believed to actas a surfactant that promotes smooth growth morphology of the III-AsNValloys. In addition, Sb can facilitate uniform incorporation of nitrogenand minimize the formation of nitrogen-related defects. Theincorporation of Sb into a III-AsNV alloy can enhance the overallnitrogen incorporation and reduce the alloy band gap, aiding therealization of lower band gap alloys. However, there are additionaldefects created by Sb and therefore it is desirable that the totalconcentration of Sb be limited to no more than 20 percent of the Group Vlattice sites. Furthermore, the limit to the Sb content decreases withdecreasing nitrogen content. Alloys that include In can have even lowerlimits to the total content because In can reduce the amount of Sbneeded to tailor the lattice constant. For alloys that include In, thetotal Sb content may be limited to no more than 3 percent of the Group Vlattice sites such as to no more than 1 percent of the Group V latticesites. For example, Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z), disclosed inU.S. Application Publication No. 2010/0319764, incorporated herein byreference, can produce a high-quality material for a dilute nitrideactive layer when substantially lattice-matched to a GaAs or Gesubstrate in the composition range of 0.08≤x≤0.18, 0.025≤y≤0.04 and0.001≤z<0.03, with a band gap of at least 0.9 eV. Further examples ofmultijunction photovoltaic cells that have dilute nitride subcells aredisclosed, for example, in U.S. Pat. Nos. 8,912,433, in 8,962,993, in9,214,580, in U.S. Application Publication No. 2017/0110613, and in U.S.Application Publication No. 2017/0213922, each of which is incorporatedby reference in its entirety, which disclose compositional rangesbetween 0≤x≤0.24, 0.001≤y≤0.07 and 0.001≤z≤0.2, and with thicknessesbetween about 1 micron and 4 microns. In some examples, multijunctionphotovoltaic cells can comprise more than one dilute nitride subcell,with each subcell having a different elemental composition and bandgap.

In some examples, a multijunction photovoltaic cell can include a dilutenitride subcell where the dilute nitride active layer comprises morethan one sub-layer, where the doping and/or composition of thesub-layers may differ. Dilute nitride sub-cells having graded dopingprofiles are disclosed, for example, in U.S. Pat. No. 9,214,580, U.S.Application Publication No. 2016/0118526, and U.S. Patent ApplicationNo, 2017/0338357, each of which is incorporated by reference in itsentirety. These publications describe dilute nitride base layerscomprising an intentionally doped region with thicknesses between 0.4microns and 3.5 microns, and with p-type doping levels between 1×10¹⁵cm⁻³ and 1×10¹⁹ cm⁻³, and further comprising an intrinsic (orunintentionally doped) diluted nitride layer or an intentionally dopeddilute nitride active layer with a constant dopant concentration, havinga thickness from 0.1 microns and about 1 micron.

The terms “layer” and “region” are used throughout the specification. A“layer” refers to a semiconductor material having a substantially singleelemental composition. A layer can be doped, and different portions of alayer can comprise different dopants and/or different dopantconcentrations. A “region” refers to a semiconductor material that cancomprise a single layer having substantially the same elementalcomposition, or a graded elemental composition, or can comprise morethan one semiconductor layer, wherein at least some of the semiconductorlayers have a different elemental composition. Each of the semiconductorlayers forming a region can be undoped or doped and the doping can varywithin a layer. For example, a “hydrogen diffusion barrier region” cancomprise one or more semiconductor layers. A “dilute nitride activelayer” refers to a single layer of semiconductor material. When referredto in the context of a photovoltaic cell, the “dilute nitride activelayer” refers to the base layer of the photovoltaic cell. A photovoltaiccell can comprise other layers such as back surface field, emitter,front surface field and window layers, which may or may not comprise adilute nitride material. A multiple quantum well structure such as avertical cavity surface emitting laser can comprise multiple dilutenitride active layers separated by semiconductor layers comprising adifferent material, and each of the layers can be grown by the sametechnique such as using MBE.

A doped region or doped layer refers to a region or doped layer that isintentionally doped. For example, a p-doped Ge layer refers to a Gelayer that has been intentionally doped with a p-type dopant. Anintentionally doped layer has a concentration of the intentional dopantthat is greater than the concentration of the dopant in the intrinsicmaterial. An undoped material can have a concentration of dopants thatare intrinsic to the deposition process and can result, for example,from impurities in the materials being deposited, backgroundcontaminants in the system, or dopants that are undesired artifacts ofthe deposition process. A material can have a concentration of anintrinsic dopant, for example, less than 10¹⁶ atoms/cm³ or less than10¹⁵ atoms/cm³. A material can have a concentration of an intentionaldopant, for example, greater than 10¹⁶ atoms/cm³, greater than 10¹⁷atoms/cm³ or greater than 10¹⁸ atoms/cm³.

Dilute nitride active layers having a bandgap that is non-uniform in thegrowth direction, orthogonal to the substrate surface are described inU.S. Provisional Application No. 62/816,718 filed on Mar. 11, 2019,which is incorporated by reference in its entirety.

Compositions and structures for dilute nitride materials for photonicdevices such as power converters and photodetectors are disclosed inU.S. Application Publication No. 2015/0221803, and in U.S. ProvisionalApplication No. 62/564,124, filed on Sep. 27, 2017, each of which isincorporated by reference in its entirety.

Dilute nitride active regions having multiple layers of dilute nitridematerials for semiconductor lasers, and in particular, vertical cavitysurface emitting lasers (VCSELs) are known, allowing emissionwavelengths between about 1.3 μm and 1.6 μm. Dilute nitride lasers andtheir compositions are described, for example, in U.S. Pat. Nos.6,798,809 and 7,645,626, and by Wistey et al., in “GaInNAsSb/GaAsvertical cavity surface emitting lasers at 1534 nm”, Electron. Lett.,42(5), 2006. Generally, quantum well materials with an In compositionbetween about 29% and 44%, N composition up to about 4% and Sbcomposition up to about 7% have been reported.

Aluminum-containing layers that can mitigate the effects of hydrogendiffusion into dilute nitride active layers are described in U.S.Application Publication No. 2019/0013429 A1, which is incorporated byreference in its entirety.

All semiconductor layers in the structures disclosed can belattice-matched or pseudomorphically strained to each of the otherlayers. “Lattice matched” refers to semiconductor layers for which thein-plane lattice constants of adjoining materials in their fully relaxedstates differ by less than 0.6% when the materials are present inthicknesses greater than 100 nm. Further, subcells that aresubstantially lattice matched to each other means that all materials inthe subcells that are present in thicknesses greater than 100 nm havein-plane lattice constants in their fully relaxed states that differ byless than 0.6%. In an alternative meaning, substantially lattice matchedrefers to the strain. Pseudomorphically strained layers can have anin-plane lattice constant that matches the lattice constant of thesubstrate, the lattice mismatch accommodated by strain within thepseudomorphic layer. As such, base layers can have a strain from 0.1% to6%, from 0.1% to 5%, from 0.1% to 4%, from 0.1 to 3%, from 0.1% to 2%,or from 0.1% to 1%; or can have strain less than 6%, less than 5%, lessthan 4%, less than 3%, less than 2%, or less than 1%. Strain refers tocompressive strain and/or to tensile strain. The thickness of apseudomorphically strained layer is less than the critical thickness forthe layer, the thickness beyond which, the strain energy can no longerbe accommodated by the layer and strain-related defects such as stackingfaults are introduced into the material. Lattice-matched andpseudomorphically strained layer are free of strain-related defects.

In certain embodiments of Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) providedby the present disclosure, the N content is not more than 7 percent ofthe Group V lattice sites. In certain embodiments, the N content is notmore than 4 percent, and in certain embodiments, not more than 3percent.

The present invention includes multijunction solar cells with three ormore subcells such as three-, four- and five-junction subcellsincorporating at least one Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell.The band gaps of the Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) materials canbe tailored by varying the content of Ga, In, N and/or As whilecontrolling the overall content of Sb. Thus, aGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell with a band gap suitable forintegrating with the other subcells of a multijunction solar cell may befabricated while maintaining substantial lattice-matching to the othersubcells. The band gaps and compositions of theGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcells can be tailored so that theshort-circuit current produced by theGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcells will be the same as orslightly greater than the short-circuit current of the other subcells inthe solar cell. Because Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) materialsprovide high quality, lattice-matched and band gap tunable subcells, thedisclosed solar cells comprising Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)subcells can achieve high conversion efficiencies. The increase inefficiency is largely due to less light energy being lost as heat, asthe additional subcells allow more of the incident photons to beabsorbed by semiconductor materials with band gaps closer to the energylevel of the incident photons. In addition, there will be lower seriesresistance losses in these multijunction solar cells compared with othersolar cells due to the lower operating currents. At higherconcentrations of sunlight, the reduced series resistance losses becomemore pronounced. Depending on the band gap of the bottom subcell, thecollection of a wider range of photons in the solar spectrum may alsocontribute to the increased efficiency.

Designs of multijunction solar cells with more than three subcells inthe prior art predominantly rely on metamorphic growth structures, newmaterials, or dramatic improvements in the quality of existing subcellmaterials in order to provide structures that can achieve highefficiencies. Solar cells containing metamorphic buffer layers may havereliability concerns due to the potential for dislocations originatingin the buffer layers to propagate over time into the subcells, causingdegradation in performance. In contrast,Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) materials can be used in solar cellswith more than three subcells to attain high efficiencies whilemaintaining substantial lattice-matching between subcells, which isadvantageous for reliability. For example, reliability testing onGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcells provided by the presentdisclosure has shown that multijunction solar cells comprise aGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell, such devices can survivethe equivalent of 390 years of on-sun operation at 100° C. with nofailures. The maximum degradation observed in these subcells was adecrease in open-circuit voltage of about 1.2%.

For applications in space, radiation hardness, which refers to minimaldegradation in device performance when exposed to ionizing radiation,including electrons and protons, is of great importance. Multijunctionsolar cells incorporating Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcellsprovided by the present disclosure have been subjected to protonradiation testing to examine the effects of degradation in spaceenvironments. Compared to Ge-based triple junction solar cells, theresults demonstrate that these Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)containing devices have similar power degradation rates and superiorvoltage retention rates. Compared to non-lattice matched (metamorphic)triple junction solar cells, all metrics are superior forGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) containing devices. In certainembodiments, the solar cells include (Al)InGaP subcells to improveradiation hardness compared to (Al)(In)GaAs subcells.

Due to interactions between the different elements, as well as factorssuch as the strain in the dilute nitride layer, the relationship betweencomposition and band gap for Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) is nota simple function of composition. The composition that yields a desiredband gap with a specific lattice constant can be found by empiricallyvarying the composition.

The thermal dose applied to the Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)material, which is controlled by the intensity of heat applied for agiven duration of time (e.g., application of a temperature of 600° C. to900° C. for a duration of between 10 seconds to 10 hours), that aGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) material receives during growth andafter growth, also affects the relationship between band gap andcomposition. In general, the band gap increases as thermal doseincreases.

As development continues on Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)materials, solar cells comprising Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)subcells, and photodetectors comprisingGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) materials, it is expected thatmaterial quality will continue to improve, enabling higher efficienciesfrom the same or similar compositions as those described in the presentdisclosure. It should be appreciated, however, that because of thecomplex interdependence of the GaInNAsSb material composition and theprocessing parameters it cannot necessarily be determined whichcombination of materials and processing conditions will produce suitablehigh efficiency subcells having a particular band gap.

As the composition is varied within theGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) material system, the growthconditions need to be modified. For example, for (Al)(In)GaAs, thegrowth temperature will increase as the fraction of Al increases anddecrease as the fraction of In increases, in order to maintain the samematerial quality. Thus, as a composition of either theGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) material or the other subcells ofthe multijunction solar cell is changed, the growth temperature as wellas other growth conditions can be adjusted accordingly.

Schematic diagrams of three junction (3J), four junction (4J), and fivejunction (5J) solar cells are shown in FIGS. 1A-1C. In some examples asshown, the solar cells may be formed on a gallium arsenide (GaAs)substrate, or on a germanium substrate. In certain embodiments, thesubstrate can comprise GaAs, InP, GaSb, (Sn,Si)Ge, or silicon. Exceptfor the experimental examples, as used herein, a germanium substraterefers to a (Sn,Si)Ge substrate, and includes, Ge, SnGe, SiGe, andSnSiGe. It can also include other substrates where the lattice constantis engineered to approximately match that of Ge, such as a bufferedsilicon substrate. Examples of buffers that can be grown on silicon toallow growth of Ge include SiGeSn, and rare-earth oxides (REOs).

In operation, a multijunction solar cell is configured such that thesubcell having the highest band gap faces the incident solar radiation,with subcells characterized by increasingly lower band gap situatedunderlying or beneath the uppermost subcell. TheGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell at the bottom of a 3J solarcell (FIG. 1A) has a band gap within a range from 0.7 eV to 1.2 eV. Theupper subcells can comprise (Al)(In)GaAs and (Al)(In)GaP, which haveprogressively higher band gaps to absorb high energy wavelengths oflight. In a 4J or higher-junction solar cell, an active germaniumsubcell lies underneath the GaInNAsSb subcell to absorb lower energy oflight. In some embodiments, such as a 5J or higher junction solar cell,two GaInNAsSb subcells with different band gaps may be used.

The specific band gaps of the subcells, are dictated, at least in part,by the band gap of the bottom subcell, the thicknesses of the subcelllayers, and the spectrum of the incident light. Although there arenumerous structures in the present disclosure that will produceefficiencies exceeding those of three junction solar cells, it is notthe case that any set of subcell band gaps that falls within thedisclosed ranges will produce an increased photovoltaic conversionefficiency. For a certain choice of bottom subcell band gap, oralternately the band gap of another subcell, the incident spectrum oflight, the subcell materials, and the subcell layer thicknesses, thereis a narrower range of band gaps for the remaining subcells that willproduce efficiencies exceeding those of other three junction solarcells.

To create a complete multijunction solar cell, other layers that may bepresent include an anti-reflection coating, contact layers, tunneljunctions, electrical contacts and a substrate or wafer handle.

Although the various layers of a multijunction solar cell can befabricated using semiconductor growth methods such as MOCVD and MBE, forcertain materials higher quality layers are preferentially grown using aparticular deposition method, such as by MOCVD or MBE. Thus, some layersof a multijunction solar cell are preferentially grown by MOVCD andother layers are preferentially grown by MBE. This can also apply tophotonic devices, including lasers such as edge-emitting lasers,vertical-cavity-surface emitting lasers (VCSELs), and photodetectorsincluding resonant-cavity photodetectors and avalanche photodetectors.MOCVD and MBE are characterized by different growth environments.

Hybrid growth of devices formed using a combination of MOCVD and MBE tofabricate individual layers or groups of layers typically requirestransfer of the semiconductor wafer and epitaxial layers from one growthenvironment to another. Consequently, a protective layer is often usedto protect the first set of epitaxial layers during transfer from onegrowth environment to another growth environment. This is done to ensurethat after the transfer the top surface of the first set of epitaxiallayers is ready for epitaxial growth. Oxidation or contamination of thegrowth layer must be prevented to make hybrid growth viable. In additionto preventing oxidation or contamination of the underlying growth layer,it is also desirable that the protective layer reduce or preventdiffusion of hydrogen during MOCVD growth into underlying dilute nitrideactive layers. It is also desirable that the protective layer withstandthermal treatments used in dilute nitride epitaxial processing. Inembodiments of the present invention, a layer comprising a dopedmaterial and/or comprising a strained layer is used as a protectivelayer overlying the at least one dilute nitride active layer (orsubcell).

FIGS. 2A-2B and FIGS. 3A-3B show simplified cross-sections of amultijunction solar cell according to embodiments of the presentdisclosure.

FIG. 2A shows an example of a four-junction cell formed on a Gesubstrate 201, which also serves as an active subcell. The Ge sub-cell201, nucleation layer 203 and a portion of the buffer layer 205 can beformed using an MOCVD process (223). The tunnel junction 207, couplingthe Ge subcell 201 to dilute nitride subcell 209, the dilute nitridesubcell 209, and at least a portion of the overlying hydrogen diffusionbarrier region 211 can be grown using an MBE process (225). Theremaining overlying layers (tunnel junction 213, subcell 215, tunneljunction 217, subcell 219 and top contact layer 221) and a portion ofthe hydrogen diffusion barrier region 211 (as required) can be grownusing an MOCVD process (227), allowing the MBE/MOCVD growth interface tobe buried in the hydrogen diffusion barrier region 227. In this example,the hydrogen diffusion barrier region 211 underlies the tunnel junction213 that couples the dilute nitride subcell 209 to the overlying subcell215. Hydrogen diffusion barrier region 211 is shown as a single layerfor simplicity. However, it will be understood that hydrogen diffusionbarrier region 211 may comprise more than one layer, with differinglayer compositions, thicknesses, doping levels and strain as describedherein.

FIG. 2B shows an example of a three-junction cell designed forconcentrated photovoltaic systems, formed on a GaAs substrate 202. Thedilute nitride subcell 204 and at least a portion of the overlyinghydrogen diffusion barrier region 206 can be grown using an MBE process(218). The remaining overlying layers (tunnel junction 208, subcell 210,tunnel junction 212, subcell 214 and top contact layer 216) and aportion of the H-diffusion barrier region 206 (as required) can be grownusing an MOCVD process (220), allowing the MBE/MOCVD growth interface tobe buried in the hydrogen diffusion barrier region 206. In this example,the hydrogen diffusion barrier region 206 underlies the tunnel junction208 that couples the dilute nitride subcell 204 to the overlying subcell210. The hydrogen diffusion barrier region 206 is shown as a singlelayer for simplicity. However, it will be understood that hydrogendiffusion barrier region 206 may comprise more than one layer, withdiffering layer compositions, thicknesses, doping levels and strain, asdescribed herein.

FIG. 3A shows an example of a four-junction cell formed on a Gesubstrate 301, which also serves as an active subcell. The Ge sub-cell301, nucleation layer 303 and a portion of the buffer layer 305 can beformed using an MOCVD process (323). The tunnel junction 307, couplingthe Ge subcell 301 to dilute nitride subcell 309, the tunnel junction311 coupling the dilute nitride subcell 309 to an overlying subcell 315and at least a portion of the overlying hydrogen diffusion barrierregion 313 can be grown using an MBE process (325). The remainingoverlying layers (subcell 315, tunnel junction 317, subcell 319 and topcontact layer 321) and a portion of the hydrogen diffusion barrierregion 313 (as required) can be grown using an MOCVD process (327),allowing the MBE/MOCVD growth interface to be buried in the hydrogendiffusion barrier region 327. In this example, the hydrogen diffusionbarrier region 313 overlies the tunnel junction 311 that couples thedilute nitride subcell 309 to the overlying subcell 315. Hydrogendiffusion barrier region 313 is shown as a single layer for simplicity.However, it will be understood that hydrogen diffusion barrier region313 may comprise more than one layer, with differing layer compositions,thicknesses, doping levels and strain as described herein.

FIG. 3B shows an example of a three-junction cell designed forconcentrated photovoltaic systems, formed on a GaAs substrate 302. Thedilute nitride subcell 304, tunnel junction 306 coupling dilute nitridesubcell 304 to overlying subcell 310, and at least a portion of theoverlying hydrogen diffusion barrier region 308 can be grown using anMBE process (318). The remaining overlying layers (subcell 310, tunneljunction 312, subcell 314 and top contact layer 316) and a portion ofthe H-diffusion barrier region 308 (as required) can be grown using anMOCVD process (320), allowing the MBE/MOCVD growth interface to beburied in the hydrogen diffusion barrier region 308. In this example,the hydrogen diffusion barrier region 308 overlies the tunnel junction306 that couples the dilute nitride subcell 304 to the overlying subcell310. The hydrogen diffusion barrier region 308 is shown as a singlelayer for simplicity. However, it will be understood that hydrogendiffusion barrier region 308 may comprise more than one layer, withdiffering layer compositions, thicknesses, doping levels and strain, asdescribed herein.

FIG. 4 shows a simplified cross-section of a photonic device accordingto an embodiment of the invention. The photonic device comprises asubstrate 401 such as GaAs, or a buffered substrate having a latticeconstant approximately equal to the lattice constant for GaAs. A firstcladding region or a first reflector such as a distributed Braggreflector (DBR) 403 can be grown using an MOCVD process. A portion oflayer 403 may also be grown using an MBE process 415. MBE process 415 isalso used to grow a dilute nitride active layer 403 and at least aportion of the overlying hydrogen diffusion barrier region 407. Theremaining overlying layers (second cladding region 409, top contactlayer 411) and a portion of the hydrogen diffusion barrier region 407(as required) can be grown using an MOCVD process (417), allowing theMBE/MOCVD growth interface to be buried in the hydrogen diffusionbarrier region 407. The overlying layer 409 can also be a secondreflector, such as a DBR. The dilute nitride active layer 405 is shownas a single layer for simplicity. However, it will be understood thatdilute nitride active layer 405 may comprise more than one layer, withdiffering layer compositions, thicknesses, doping levels and strain, asdescribed herein.

In some embodiments, dilute nitride active layer 405 can include adilute nitride material. The dilute nitride material can beGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) where x, y and z can be 0≤x≤0.4,0<y≤0.07 and 0<z≤0.2, respectively. In some embodiments, x, y and z canbe 0.01≤x≤0.4, 0.02≤y≤0.07 and 0.001≤z≤0.04, respectively, and thethickness of the dilute nitride active layer can be within a range from0.2 μm to 10 μm or from 1 μm to 4 μm. In some embodiments, active layer405 can include quantum well structures to form a dilute nitride activelayer. For example, dilute nitride active layer can include GaInNAs/GaAsor GaInNAsSb/GaAsN multiple quantum wells (MQWs). Examples of dilutenitride-based active layers, including compositions and thicknesses, aredescribed in U.S. Pat. Nos. 6,798,809, and 7,645,626, each of which isincorporated by reference in its entirety. The hydrogen diffusionbarrier region 407 is shown as a single layer for simplicity. However,it will be understood that hydrogen diffusion barrier region 407 maycomprise more than one layer, with differing layer compositions,thicknesses, doping levels and strain, as described herein.

Devices according to FIG. 4 can include VCSELs, resonant cavity enhancedphotodetectors (RCEPDs), edge-emitting lasers (EELs), light emittingdiodes (LEDs), photodetectors and modulators. In some examples, hydrogendiffusion barrier region 407 can also be a reflector structure and canform part of an overlying distributed Bragg reflector. Hydrogendiffusion barrier region 407 can comprise, for example, semiconductormaterials of Groups III and V of the periodic table such as, forexample, AlAs, AlGaAs, GaAs, InAs, InGaAs, AlInAs, InGaP, AlInGaP,InGaP, GaP, InP, AlP, AlInP, or AlInGaAs. An example of a reflector is aDBR. A DBR may comprise, for example, a plurality of alternatingAlGaAs/GaAs layers, with thicknesses selected to provide a desiredreflectivity over a given wavelength range and incident angle. Otherreflector designs may also be used. A first portion of the DBR can beformed using an MBE process 415 (to act as the hydrogen diffusionbarrier) and a second adjacent overlying portion of the DBR can beformed using an MOCVD process 417. In other examples, hydrogen diffusionbarrier region 407 can also be a waveguiding layer and/or a claddingregion.

Returning to multijunction solar cells, FIGS. 5A-5D show examples of a4J solar cell structures with the previously-mentioned additionalelements. Further, additional elements may be present in a completesolar cell, such as buffer layers, tunnel junctions, back surface fieldlayers, window layers, emitters, nucleation layers, and front surfacefield layers. In the examples shown in FIGS. 5A-5D all subcells can besubstantially lattice-matched to each other and may be interconnected bytunnel junctions. Multijunction solar cells may also be formed withoutone or more of the elements listed above.

In each of the embodiments described herein, the tunnel junctions aredesigned to have minimal light absorption. Light absorbed by tunneljunctions is not converted into current by the solar cell, and thus ifthe tunnel junctions absorb significant amounts of light, it will not bepossible for the efficiencies of the multijunction solar cells to exceedthose of the best multijunction junction solar cells. Accordingly, thetunnel junctions must be very thin (for example, less than 40 nm) and/orbe made of materials with band gaps equal to or greater than thesubcells immediately above the respective tunnel junction. An example ofa tunnel junction fitting these criteria is a GaAs/AlGaAs tunneljunction, in which each of the GaAs and AlGaAs layers forming the tunneljunction has a thickness between 5 nm and 40 nm. The GaAs layer can bedoped with Te, Se, S and/or Si, and the AlGaAs layer can be doped withC. InGaAs may also be used in the tunnel junctions instead of GaAsand/or AlGaAs. Another example of a suitable tunnel junction is aGaInP/AlGaAs tunnel junction, with similar thicknesses, and where theInGaP layer can be doped with Te, Se, S and/or Si, and the AlGaAs layercan be doped with C.

Various metrics can be used to characterize the quality of anoptoelectronic device, including, for example, the Eg/q-Voc, theefficiency over a range of irradiance energies, the open circuitvoltage, Voc, and the short circuit current density, Jsc. Those skilledin the art can understand how to extrapolate the Voc and Jsc measuredfor a junction having a particular dilute nitride base thickness toother junction thicknesses. The Jsc and the Voc are the maximum currentdensity and voltage, respectively, for a solar cell. However, at both ofthese operating points, the power from the solar cell is zero. The fillfactor (FF) is a parameter which, in conjunction with Jsc and Voc,determines the maximum power from a solar cell. The FF is defined as theratio of the maximum power produced by the solar cell to the product ofVoc and Jsc. Graphically, the FF is a measure of the “squareness” of thesolar cell and is also the area of the largest rectangle, which will fitwithin the IV (current-voltage) curve.

Seemingly small improvements in the efficiency of a junction/subcell canresult in significant improvements in the efficiency of a multijunctionsolar cell. Again, seemingly small improvements in the overallefficiency of a multijunction solar cell can result in dramaticimprovements in output power, reduce the area of a solar array, andreduce costs associated with installation, system integration, anddeployment.

Solar cell efficiency is important as it directly affects the solarmodule power output. For example, assuming a 1 m² solar panel having anoverall 24% conversion efficiency, if the efficiency of multijunctionsolar cells used in a module is increased by 1% such as from 40% to 41%under 500 suns, the module output power will increase by about 2.7 kW.

Generally, a solar cell contributes around 20% to the total cost of asolar power module. Higher solar cell efficiency means morecost-effective modules. Fewer solar devices are then needed to generatethe same amount of output power, and higher output power with fewerdevices leads to reduced system costs, such as costs for mounting racks,hardware, wiring for electrical connections, etc. In addition, by usinghigh efficiency solar cells to generate the same power, less land area,fewer support structures, and lower labor costs are required forinstallation.

Solar modules are a significant component in spacecraft power systems.Lighter weight and smaller solar modules are always preferred becausethe lifting cost to launch satellites into orbit is super expensive.Solar cell efficiency is especially important for space powerapplications to reduce the mass and fuel penalty due to large arrays.The higher specific power (watts generated over solar array mass), whichdetermines how much power one array will generate for a given launchmass, can be achieved with more efficient solar cells because the sizeand weight of the solar array will be less for the same power output.

As an example, compared to a nominal solar cell having a 30% conversionefficiency, a 1.5% increase in multijunction solar cell efficiency canresult in a 4.5% increase in output power, and a 3.5% increase inmultijunction solar cell efficiency can result in an 11.5% increase inoutput power. For a satellite having a 60 kW power requirement, the useof higher efficiency subcells can result in solar cell module costsavings from $0.5 million to $1.5 million, and a reduction in solararray surface area of 6.4 m² to 15.6 m², for multijunction solar cellshaving increased efficiencies of 1.5% and 3.5%, respectively. Theoverall cost savings will be even greater when costs associated withsystem integration and launch are taken into consideration.

One important problem to solve in making hybrid epitaxy viable is thepotential oxidation or contamination of exposed interface layers asepitaxial growth is interrupted and epi-wafers are moved from onereactor to another. Any imperfections at the interface where growthresumes will result in poor overgrown epitaxial material. In production,cluster tools and controlled-atmosphere boxes are employed. A possiblesolution is the careful design of the epitaxial stack so thatinterruption of epitaxy occurs at layers that form a protective cap fromoxidation. Consider this in the context of the 4-junction (4J)lattice-matched solar cell (FIGS. 1B, 3A, 5A), where the epitaxialmaterial is grown on a p-doped germanium substrate that is oriented inan-off axis crystallographic direction. The bottom junction (or J4subcell) is created in the germanium substrate using a MOCVD growthtechnique. The top layer of this J4 structure is a 30 nm- to 150nm-thick layer of n-doped (In)GaAs (buffer layer), which protects thisepiwafer from oxidation as it is removed from the MOCVD reactor andinserted into an MBE chamber for growth of the dilute nitride junction(J3). Epitaxial growth resumes in the MBE chamber by completing then-doped (In)GaAs buffer layer, then growing a tunnel junction, backsurface field and a layer of dilute nitride for J3.

A layer of (In)GaAs and/or (Al)GaAs can be grown over the dilute nitrideactive layer as an emitter layer, which can also serve as a protectivecap layer before the epiwafer is removed from the MBE and loaded into anMOCVD to finish the growth of junctions J2 and J1. (In)GaAs and (Al)GaAsare materials that can also function as a tunnel junction, aback-surface field (BSF), a front surface field (FSF), a window layerand an emitter layer. Consider this in the context of the 4Jlattice-matched solar cell shown in FIG. 5A; after growth of theGaInNAsSb base by MBE, an InGaAs emitter and an InGaAs tunnel junctioncan be grown over the GaInNAsSb base. After that, a hydrogen diffusionbarrier region can be grown over the emitter and tunnel junction layers.In some embodiments, the hydrogen diffusion barrier region comprises amaterial that can be used as a protective layer over the GaInNAsSbbefore the epiwafer is transferred from the MBE to the MOCVD, andepitaxial growth can be interrupted after growth of at least a portionof, or all of the hydrogen diffusion barrier region by MBE, andepitaxial growth can be interrupted after growth of this protectivelayer. In some embodiments, an additional protective layer can bedeposited on the hydrogen diffusion barrier region. In some embodiments,the hydrogen diffusion barrier region comprises a doped material. Insome embodiments, the hydrogen diffusion barrier region comprises apseudomorphically strained layer. Once the epiwafer is in the MOCVD,growth of subsequent layers may continue.

The hydrogen diffusion barrier region mitigates the effects of hydrogendiffusion into the underlying dilute nitride material during growth ofthe overlying layers in a MOCVD environment. Hydrogen is known to act asan isolated donor in dilute nitrides, can intentionally passivatedopants by forming complexes with the intentional dopants, and is alsoknown to form complex defects of nitrogen and hydrogen, such as N—H, andN—H-VGa. The presence of hydrogen in dilute nitrides can thereforeaffect the electrical and optical properties of dilute nitrides. The useof a hydrogen diffusion barrier region prevents the creation of dopingpassivation-compensation, isolated donors, and/or other defects in thedilute nitride material.

The hydrogen diffusion barrier region can be formed immediately abovethe tunnel junction (FIG. 3A, FIG. 3B, FIG. 5A, FIG. 5B) or immediatelybelow a tunnel junction (FIG. 2A, FIG. 2B, FIG. 5C, FIG. 5D).

A hydrogen diffusion barrier region can be a single layer or can be morethan one layer where each layer can have the same alloy composition orcan have different alloy compositions, which may be deposited under thesame or different growth conditions. The layers can have the same ordifferent doping levels and/or the same or different strain values. Insome embodiments, the hydrogen diffusion barrier region can be adjacentthe dilute nitride active layer. In other embodiments, the hydrogendiffusion barrier region in not adjacent the dilute nitride activelayer, for example, as will be described in the embodiment shown in FIG.9. “Adjacent” refers to a semiconductor layer that is directly on ordirectly grown on another semiconductor layer.

The dilute nitride active layer or layers, the one or more hydrogendiffusion barrier regions, and any intervening layers between the activelayer or layers and the one or more hydrogen diffusion barrier regionscan be grown by MBE and the overlying semiconductor layer can be grownby MOCVD. The overlying semiconductor layer can comprise GaAs, AlGaAs,InGaAs, InAlP, AlGaInP or InGaP.

Embodiments for the hydrogen diffusion barriers, including compositions,thicknesses, doping concentrations and strain levels will be describedin more detail later.

Referring to FIG. 5A in more detail, a p-doped germanium substrate 502is provided and cleaned to remove native oxides prior to atomic-layerdeposition. The substrate may be cleaned, for example, in a gaseousenvironment such as an AsH₃ environment or a PH₃ environment. Thiscleaning step also allows the group V atoms to diffuse into the upperregion of germanium. An emitter region is formed as the upper germaniumregion and is doped with a group V element comprising arsenic orphosphorus, transforming the germanium substrate into an active n-pjunction, with a p-doped region and an n-doped region overlying thep-doped region. In the four-cell embodiment shown in FIG. 5A, this cellis referred to as “J4”. The extent of group-V diffusion can beinfluenced by thermal exposure during substrate cleaning, epitaxialgrowth, and post-growth annealing treatments. In some embodiments, aphosphide layer or an arsenide layer can be deposited on the top surfaceof substrate 502, with the deposition conditions allowing for diffusionof the group V atoms into substrate 502 to form the n-doped region.

A nucleation layer 504 can be epitaxially grown over the p-dopedgermanium junction 502. A nucleation layer 504 is epitaxially grown onthe top surface of the germanium junction 502. The nucleation layer canbe, for example, InGaP. However, other nucleation layers are known andare disclosed, for example, in U.S. Pat Nos. 6,380,601 B1 and in7,339,109 B2, although the nucleation layers disclosed in thesepublications were not applied to dilute-nitride-based multijunctioncells. Nucleation layers used with dilute nitride materials aredescribed in U.S. Application Publication No. 2018/0053874 and in U.S.Application Ser. No. 16/276,432 filed on Feb. 14, 2019, each of which isincorporated by reference in its entirety. Nucleation layers cancomprise, for example, InGaP, InGaPSb, InAlP, AlP, AlPSb, GaPSb,AlGaPSb, InAlPSb, InAlPBi, InAlPSbBi, AlInGaP, AlInGaPSb, AlInGaPBi,AlInGaPSbBi, AlP, AlPSb, AlPBi, AlPSbBi, AlAsSb, AlAsBi, AlAsSbBi, AlN,AlNSb, AlNBi, or AlNSbBi. Nucleation layer 504 can have a thickness, forexample less than 200 nm, less than 100 nm, less than 50 nm, less than20 nm, less than 10 nm, or less than 1 nm. Nucleation layer 304 can be,for example, from 2 nm to 20 nm thick, from 2 nm to 10 nm, from 2 nm to5 nm, or from 4 nm to 10 nm. A nucleation layer 504 can be n-doped. Abuffer layer 506 can then be epitaxially grown over nucleation layer504. A buffer layer can comprise (In)GaAs. A (In)GaAs buffer layer canbe, for example, from 100 nm to 900 nm thick, from 200 nm to 800 nmthick, from 300 nm to 700 nm thick, or from 400 nm to 600 nm thick. Abuffer 506 can be n-doped. The nucleation layer and at least a portionof the buffer layer can be grown using MOCVD.

In some embodiments, after growth of at least a portion of the bufferlayer 506, the sample can be transferred to an MBE chamber forsubsequent growth of the dilute nitride sub-cell or junction. In someembodiments, buffer layer 506 may be completed by growing severalnanometers (e.g., 2 nm to 100 nm) of (In)GaAs using MBE, prior todeposition of the subsequent layers to ensure that a growth interruptoccurs in the buffer layer and is not directly adjacent to a structuresuch as the overlying tunnel junction. After completion of the bufferlayer, a tunnel junction 508 can then be epitaxially grown over bufferlayer 506. Tunnel junction 508 can comprise two InGaAs layers, with thefirst layer 508A having a high n-type doping level, and the second layer508B having a high p-type doping level. Compositions, thicknesses anddoping levels required to form tunnel junctions are known in the art.For example, n-dopants can include Si, Se, and Te and n-type dopinglevels can range from 1×10¹⁹ cm⁻³ to 2×10²⁰ cm⁻³. P-type dopants caninclude C and doping levels greater than about 1×10¹⁹ cm⁻³ and up to2×10²⁰ cm⁻³ can be used. Thicknesses for the doped layers in tunneljunctions can be between about 5 nm and 40 nm.

Sub-cell 501 (referred to as “J3”) is then epitaxially deposited ontunnel junction 508. Sub-cell 501 comprises a p-doped InGaAs backsurface field layer 510, a p-doped GaInNAsSb base layer 512A, anintrinsic or unintentionally doped base layer 512B and an n-doped InGaAsemitter layer 514. The p-doped layer 512A and layer 512B can comprise,individually, Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z), with 0≤x≤0.24,0.001≤y≤0.07 and 0.001≤z≤0.2, or with 0.08≤x≤0.24, 0.02≤y≤0.05 and0.001≤z≤0.02, or with 0.07≤x≤0.18, 0.025≤y≤0.04 and 0.001≤z≤0.03, orwith 0≤x≤0.4, 0<y≤0.07, and 0<z≤0.04. The p-doped base layer 512A canhave a graded doping profile, with the doping level decreasing from theinterface with back surface field 310 to the interface with base layer512B. The intentional doping in base layer 512B can be gradedexponentially between 1×10¹⁹ cm⁻³ and 1×10¹⁵ cm⁻³, for example between1×10¹⁸ and 5×10¹⁵ cm⁻³, or between 2×10¹⁷ and 7×10¹⁵ cm⁻³, where theminimum doping level is greater than or equal to the background dopinglevel of the base layer. Base layer 512B can be an intrinsic layer or anunintentionally doped base layer, with a background doping concentrationless than about 1×10¹⁶ cm⁻³ or less than about 5×10¹⁵ cm⁻³ or less thanabout 1×10¹⁵ cm⁻³. Base layer 512B can also be doped at a fixed dopinglevel of 1×10¹⁶ cm⁻³ or less. Sub-cell 501 can have a thickness betweenabout 1 micron and 4 microns.

A tunnel junction 516 can then be epitaxially grown over sub-cell 501.Tunnel junction 516 comprises two InGaAs layers, one with high p-typedoping, the other with high n-type doping. Compositions, thicknesses anddoping levels used to form tunnel junctions are known in the art. Forexample, typical n-dopants include Si, Se, and Te and n-type dopinglevels can range between 1×10¹⁹ cm⁻³ and up to 2×10²⁰ cm⁻³. P-typedopants include C and doping levels greater than 1×10¹⁹ cm⁻³ and up to2×10²⁰ cm⁻³ can be used. Thicknesses for the doped layers in tunneljunctions can be between about 5 nm and 40 nm.

A hydrogen diffusion barrier region 507A can then be epitaxially formedon tunnel junction 516. FIG. 5A shows layer 507A as a single layer.However, it will be understood that hydrogen diffusion barrier region318 may include more than one material layer. In some embodiments,hydrogen diffusion barrier region 507A comprises an Al-containing layer.In some embodiments, hydrogen diffusion barrier region 507A comprises adoped layer. In some embodiments, hydrogen diffusion barrier region 507Acomprises a pseudomorphically strained layer. In some embodiments,hydrogen diffusion barrier region 507A is capped by a layer of GaAs,InGaAs or InGaP, having a thickness, for example, between 1 nm and 50nm, or between 2 nm and 10 nm, or between 2 nm and 5 nm. In someembodiments, the hydrogen diffusion barrier can also function as a BSFlayer for an overlying subcell in a multijunction solar cell (such aslayer 518). Embodiments for the hydrogen diffusion barrier regions,including compositions, thicknesses, doping concentrations and strainlevels will be described in more detail.

After growth of layer 507A, or at least a portion of layer 507A, thewafer is transferred to an MOCVD chamber for deposition of the remainderof layer 507A (if desired) and the remaining layers to complete themultijunction cell. Sub-cell 503 (referred to as “J2”) is thenepitaxially formed on hydrogen diffusion barrier region 507A. Sub-cell503 comprises an Al-containing back surface field layer 518. Forexample, layer 518 can comprise AlGaAs, or InAlP and can be latticematched or pseudomorphically strained to the substrate. Sub-cell 503 iscompleted by deposition of base 520, emitter 522 and front surface fieldlayer 524. A tunnel junction 526 is then epitaxially grown.Compositions, thicknesses and doping levels used to form tunneljunctions are known in the art. By way of example, tunnel junction 526is shown comprising a GaAs layer and an AlGaAs layer. However, it willbe understood that other materials may be used. For example, the tunneljunction may comprise an InGaP layer and/or an AlGaAs layer. Examples ofn-dopants for the tunnel junction layers include Si, Se, and Te andn-type doping levels in a range between 1×10¹⁹ cm⁻³ and up to 2×10²⁰cm⁻³ can be used. P-type dopants can include C and doping levels in arange between 1×10¹⁹ cm⁻³ and up to 2×10²⁰ cm⁻³ can be used. Thicknessesfor the doped layers in tunnel junctions can be between 5 nm and 40 nm.Sub-cell 505 (J1) is then epitaxially grown, depositing in sequence backsurface field layer 528, base layer 530, emitter layer 532, frontsurface field layer 534, and contact layer 336.

FIG. 5B shows an alternative implementation of a hydrogen diffusionbarrier region. After completion of growth of tunnel junction 516, areflector region 507B is deposited. Reflector region 507 is shown as asingle layer. However, it will be understood that reflector layer caninclude one or more layers with differing compositions, thicknesses anddoping levels in order to provide the appropriate optical and/orelectrical functionality, and to improve interface quality, electrontransport, hole transport and/or other optoelectronic properties.Reflector region 507 can comprise alternating layers of materials havingdifferent refractive indices. The refractive index difference betweenthe layers, and the layer thicknesses provides a reflectivity over adesired wavelength range. Reflector region 507 comprises at least twodifferent materials with different refractive indices and at least twodifferent layer thicknesses. In some embodiments, the mirror materialsare doped. In some embodiments, at least one material ispseudomorphically strained. Reflector region 507 can comprise, forexample semiconductor materials of Groups III and V of the periodictable such as, for example, AlAs, AlGaAs, GaAs, InAs, InGaAs, AlInAs,InGaP, AlInGaP, InGaP, GaP, InP, AlP, AlInP, or AlInGaAs. An example ofa reflector is a DBR. A DBR may comprise, for example, a plurality ofalternating AlGaAs/GaAs layers, with thicknesses selected to provide adesired reflectivity over a given wavelength range and incident angle.Other reflector designs may also be used. Reflector region 507 can bedesigned to reflect wavelengths absorbed by the overlying sub-cell 503.Reflector region 507 also serves as a hydrogen diffusion barrier. Atleast a portion of reflector region 507 can be grown by MBE, after whichthe epiwafer is transferred to an MOCVD chamber. In some embodiments,reflector region 507 may be completed by MOCVD growth, after whichoverlying back surface field 518 and the remaining layers can be grownvia MOCVD. Further details of embodiments for the hydrogen diffusionbarrier region, including compositions, thicknesses, dopingconcentrations and strain levels will be described more fully later.

The hydrogen diffusion barrier region may also underlie the InGaAstunnel junction 516, as shown in FIG. 5C. In this embodiment, a windowlayer 509 grown by MBE overlies emitter 514 and underlies tunneljunction 516. Window region 509 is shown as a single layer. However, itwill be understood that window region 509 may include more than onematerial layer. Window region 509 is configured to function as ahydrogen diffusion barrier region. In some embodiments, window region509 comprises an Al-containing layer. In some embodiments, window region509 comprises a doped layer. In some embodiments, window region 509comprises a pseudomorphically strained layer. In some embodiments,window region 509 is capped by a layer of GaAs, InGaAs or InGaP, havinga thickness, for example, between 1 nm and 50 nm, or between 2 nm and 10nm, or between 2 nm and 5 nm. Embodiments for the hydrogen diffusionbarriers, including compositions, thicknesses, doping concentrations andstrain levels will be described in more detail.

After MBE growth of window layer 509, or at least a portion of windowregion 509, MOCVD growth can be used to complete the window layer (ifdesired) and for the remaining layers of the device.

FIG. 5D shows yet another embodiment of a hydrogen diffusion barrierunderlying tunnel junction 516. In this example, emitter region 514shown in FIG. 5A is replaced by emitter region 515. comprising ahydrogen diffusion barrier region. Emitter region 509 is shown as asingle layer. However, it will be understood that emitter layer 515 mayinclude more than one material layer. Emitter region 515 is configuredto function as a hydrogen diffusion barrier. In some embodiments,emitter region 515 comprises an Al-containing layer. In someembodiments, emitter region 515 comprises a doped layer. In someembodiments, emitter region 515 comprises a pseudomorphically strainedlayer. In some embodiments, emitter region 515 is capped by a layer ofGaAs, InGaAs or InGaP, having a thickness, for example, between 1 nm and50 nm, or between 2 nm and 10 nm, or between 2 nm and 5 nm. Embodimentsfor the hydrogen diffusion barrier regions, including compositions,thicknesses, doping concentrations and strain levels will be describedin more detail.

MBE can be used to grow at least a portion of emitter region 515. MOCVDgrowth can be used to complete growth of emitter region 515 (if desired)and for the remaining layers of the device.

The dilute nitride material is a high-efficiency solar material only ifit is processed in particular ways. For example, thermal treatment isneeded to activate the material. It is not trivial to determine thespecific process step(s) during which thermal treatment must be appliedfor annealing.

In general, a thermal treatment, such as a rapid thermal treatment (RTA)refers to exposure to a temperature that can range from 600° C. to 900°C. for a duration from 5 seconds to 3 hours. In some cases, there are nolimits for temperature and time. Table 1 summarizes typical thermaltreatment parameters by deposition method or thermal annealingcondition.

TABLE 1 Thermal treatment methods, temperatures and exposure times. Ovenor Method MBE MOCVD RTA furnace Time 2-3 hours 0.5-1 hour 0.1-10 minutesAny duration Temper- 600° C.- 630° C.- 600° C.- Any temperature ature650° C. 700° C. 900° C.

Thermal treatment can damage the surface morphology of a dilute nitrideactive layer such as a J3 subcell or other dilute nitride active layersuch as a bulk layer or a quantum well layer, which has to be ofsufficient quality for additional epitaxial growth in the MOCVD reactor.Although it is possible to thermally treat a dilute nitride active layerprior to subsequent epitaxial growth (FIG. 6A) or after all epitaxialgrowth is completed (FIG. 6B), it is unclear which practice is best toproduce a high efficiency device. An increase in haze post-thermaltreatment is not uncommon and is indicative of structural defects.Thermal treatment prior to MOCVD growth will create areas with haze inthe dilute nitride active layer, which can nucleate structural defectsin subsequent epitaxial layers grown on the dilute nitride active layer.These defects can propagate throughout the device structure and therebydecrease device performance. Besides, exposure of the growing epi-waferin the MOCVD reactor can provide sufficient thermal load to activate thedilute nitride active layer, which would render additional thermaltreatment redundant.

An additional consideration for multijunction solar cells grown on anactive germanium substrate (FIG. 1) is to design the structure of theupper junctions and to use the growth conditions such that the thermalload is maintained below a threshold that otherwise would cause thebottom dilute nitride subcell (or subcells) to degrade due to excessdiffusion of phosphorus into the active germanium substrate. Thisdegradation reduces open circuit voltage (Voc) and the conversionefficiency of the bottom germanium junction. Limiting the thermal loadduring growth of the upper junctions can be accomplished by acombination of one or more of the following measures: (i) reducing thegrowth temperature, (ii) reducing the growth time by increasing growthrates; and (iii) reducing the growth time by reducing the thickness ofsome of the layers in the upper junctions.

Another consideration in multijunction solar cells grown on a germaniumor GaAs substrate (FIG. 1) is to design the epitaxial structure and theepitaxial growth conditions of the J2 and J1 junctions above the dilutenitride junction (which is J3 in the 4J solar cell on germaniumembodiment, and J3 on the 3J solar cell on GaAs embodiment) to applyenough thermal load during the MOCVD growth of J2 and J1 to fully annealthe dilute nitride junction. For photonic devices, epitaxial growthconditions for an overlying DBR or cladding region can be chosen toapply enough thermal load during the MOCVD growth of the overlying DBRto fully anneal the dilute nitride active layer. In this case, thedilute nitride active layer is fully annealed in situ during the MOCVDgrowth of the overlying layers and no additional ex-situ thermaltreatment is required (FIG. 6E). Delivering the appropriate thermal loadto adequately anneal the dilute nitride active layer during the MOCVDgrowth of the overlying layers can be accomplished by a combination ofone or more of the following measures: (i) reducing the growthtemperature during the MOCVD growth of upper junction, (ii) reducing theMOCVD growth time by increasing growth rates; and (iii) reducing theMOCVD growth time by reducing the thickness of some of the layers in theupper junctions. Over-annealing the dilute nitride active layer byexcessive thermal load during growth of overlying layers such as J2 andJ1 or a DBR region will not only degrade the dilute nitride active layerbut in the case of a solar cell on germanium, it will also degrade thebottom junction.

As an alternative, one can also apply sufficient thermal load during theMOCVD growth of the overlying layers (J2 and J1 subcells or DBR) topartially or under-anneal the dilute nitride active layer. Deliveringthe appropriate thermal load to partially or under-anneal the dilutenitride active layer in-situ during the MOCVD growth of the upperjunctions can be accomplished by a combination of one or more of theaforementioned measures that pertain to growth temperatures, rates andtimes. An additional thermal anneal can be performed ex situ after allepitaxial growth is completed, using one of several possible methodsincluding, for example, RTA (rapid thermal anneal), oven baking, or tubefurnace annealing.

Additional process flows for forming devices are shown in FIG. 6C andFIG. 6D, where thermal treatment steps may be interleaved with epitaxialgrowth steps.

As mentioned previously, one solution to hydrogen contamination is toprotect the dilute nitride with a barrier. Hydrogen getters arematerials capable of binding hydrogen gas under low pressure (less than1 atm) and can be incorporated in the design of a multijunction solarcell. Although it is common practice to liberate absorbed hydrogen byapplying thermal treatment post-epitaxial growth (FIGS. 6B-6D,), doingso can further worsen the doping profiles already altered by hydrogendiffusion, resulting in even poorer device performance. In anyembodiment of a multijunction solar cell that has a layer of dilutenitride, a hydrogen getter material caps the dilute nitride before theepiwafer leaves the low-hydrogen environment of the MBE. Once in theMOCVD reactor, the hydrogen getter preserves the quality of theunderlying dilute nitride by absorbing hydrogen gas on its surface.

Embodiments and examples for the hydrogen diffusion barrier regions willnow be described in more detail.

A hydrogen diffusion barrier region can have a thickness, for example,from 25 nm to 6 μm, from 50 nm to 4 μm, from 100 nm to 2 μm, from 100 nmto 1 μm, from 100 nm to 500 nm, or from 1 μm to 3 μm.

In certain embodiments, at least one aluminum-containing layer is usedas the hydrogen diffusion barrier. Examples of suitablealuminum-containing layers include AlGaAs, AlGaAsSb, AlGaAsBi, AlInP,AlInGaP, AlInGaPSb, AlInGaPBi, AlInGaAs, AlInGaAsSb, AlInGaAsBi, AlN,AlNSb, and AlNBi. A thin layer of aluminum material can be used if thealuminum content is sufficiently high, and a thick layer of aluminummaterial can be used if the percentage of aluminum is low. The at leastone aluminum-containing layer can comprise AlGaAs or InAlP. In someexamples, AlGaAs is used as a hydrogen diffusion barrier. Using AlGaAsas an example, the thickness of AlGaAs ranges from 100 nm to 5 microns,such as, for example, from 100 nm to 2 μm, from 100 nm to 1 μm, from 100nm to 500 nm, or from 100 nm to 200 nm. AlGaAs can comprise, forexample, Al_(x)Ga_(1-x)As with an aluminum content where 0.05<x≤1, suchas 0.05<x≤0.8, or 0.05<x≤0.6, or 0.05<x≤0.5, or 0.05<x≤0.4, 0.05<x≤0.3,or 0.1<x≤0.4. An aluminum-containing material can comprise, for example,from 5 mol % to 100 mol % of aluminum, from 10 mol % to 80 mol %, from20 mol % to 60 mol %, from 25 mol % to 55 mol %, from 30 at% to 50 mol%, or from 35 mol % to 45 mol %, where mol % is based on the fraction ofgroup-III atoms in the aluminum-containing material.

An aluminum-containing reflector region can comprise at least twodifferent materials with different refractive indices and at least twodifferent layer thicknesses, with at least one material comprising Al.The aluminum-containing reflector region can comprise, for examplesemiconductor materials of Groups III and V of the periodic table suchas, for example, AlAs, AlGaAs, GaAs, InAs, InGaAs, AlInAs, InGaP,AlInGaP, InGaP, GaP, InP, AlP, AlInP, or AlInGaAs, with at least onelayer of the mirror layers comprising Al. An aluminum-containingreflector region can have a thickness, for example, from 100 nm and 6microns.

An aluminum-free layer can overlie and be adjacent to analuminum-containing layer. The aluminum-free layer can have a thickness,for example, from 1 nm to 200 nm, from 1 nm to 100 nm, from 5 nm to 75nm, from 10 nm to 50 nm, or from 10 nm to 30 nm. An aluminum-free layercan comprise, for example, GaP, GaAs, InGaP, GaAsP, or InGaAsP, InGaPSbor InGaAsSb. The aluminum-free layer can be a functional layer such as,for example, a window layer or a mirror layer or an emitter layer of asolar cell.

In certain embodiments, a hydrogen diffusion barrier region does notcomprise an aluminum-containing layer, such as GaP, GaAs, InGaP, GaAsP,InGaAsP, InGaAsSb or InGaPSb.

In other embodiments, at least one doped layer is used as the hydrogendiffusion barrier. P-type dopants include C, Be and Zn. N-type dopantsinclude Si, Se and Te. The incorporation of dopants into III-V compoundsemiconductor devices can introduce defects. Furthermore, it is knownthat dopants and defects can form complexes with H. Complexing with adopant can form electrically inactive centers. This behavior in a layeroverlying a dilute nitride active layer can result in the hydrogendiffusion barrier region effectively gettering the hydrogen, preventingdiffusion into the underlying dilute nitride active layers, providedthat a suitable doping concentration and layer thickness is used. In oneembodiment, carbon doping is used. Carbon preferentially substitutes forAs in materials such as GaAs, AlGaAs and InGaAs, and its low diffusivityand high solubility make it an ideal dopant, with accepterconcentrations as high as several 10²⁰ cm⁻³ achievable using MBE growth.Hydrogen is known to passivate C dopants, forming C_(As)—H pairs. Dopinglevels in a range between about 10¹⁶ cm⁻³ and 2×10²⁰ cm⁻³ or between10¹⁷ cm⁻³ and 10¹⁹ cm⁻³ with layer thicknesses between 50 nm and 6 μm,or between 100 nm and 5 μm or between 200 nm and 3 μm or between 500 nmand 2 μm can be used to provide a total number of dopants within thehydrogen diffusion barrier region. In some embodiments, the dopingconcentration can be graded within the layer. Hydrogen has been observedto passivate up to 60% of C. Passivation of other dopants and defectshas also been observed and can also be used in a similar fashion. Thus,by including a suitable concentration of dopants, hydrogen can begettered within the hydrogen diffusion barrier region, reducing oreliminating hydrogen diffusion into the underlying dilute nitride activelayers, while allowing a sufficient active doping level to remain thatallows electrical functionality to be preserved within the dilutenitride active layer. Compositions for the doped hydrogen diffusionbarrier layer can include GaAs, GaAsP, InGaAs, GaP, InGaP, InGaAsP, AlP,InAlP, or AlGaInP.

In other embodiments, at least one nitrogen-containing layer is used asthe hydrogen diffusion barrier. Nitrogen containing layers includeGaAsN, AlGaAsN, GaInAsN, GaN, AN, AlNSb, GaNSb, GaNAsSb, GaInNAsSb,GaNBi and AlNBi. The bandgap for the nitrogen-containing layer is largerthan the bandgap of the dilute nitride active layer or layers, such as adilute nitride base layer of a photovoltaic subcell, or a dilute nitridequantum well. A nitrogen-containing material can comprise, for example,from 0.5 mol % to 3 mol % of nitrogen, or from 1 mol % to 2 mol %, wheremol % is based on the fraction of group-V atoms in thenitrogen-containing material. In a photonic device such as a laser,nitrogen can be included in the waveguiding or cladding region overlyingan active dilute nitride quantum well region, preventing hydrogendiffusion into the quantum wells. In a solar cell, a nitrogen-containinglayer can form a window layer or an emitter layer overlying a dilutenitride base layer (absorbing region). The presence of small levels ofnitrogen within a material can affect the local potential in thematerial as well as introduce defects which are known to form complexeswith hydrogen, thus preventing the diffusion of H into the underlyingdilute nitride active layer that provides the optical and electricalfunctionality for the device.

In other embodiments, at least one strained layer is used as thehydrogen diffusion barrier. U.S. application Ser. No. 16/276,432 filedon Feb. 14, 2019, provides nucleation layers grown on Ge that alsoprevent the diffusion of group V elements into the Ge substrate. Theselayers comprise InAlPSb, where bond-strength and strain effects (such asin AlP/InAlP layers) are believed to control the level of hydrogendiffusion. To be able to prevent hydrogen diffusion, different strainedstructures can be used, for example, strained layer superlattices(SLSs). FIG. 7 shows a cross section of a SLS comprising two alternatinglayers. SLS 700 comprises adjacent layers of a first layer 702, with afirst composition and a first thickness t₇₀₂, and a second layer 704with a second composition and a second thickness t704. One layer is incompressive strain, while the other is in tensile strain. Thecomposition of the adjacent layers is different. M pairs of layers canbe used to form the superlattice, where M is an integer determined bythe thickness of the layer pair and the total desired thickness of thesuperlattice. The strain per layer pair is minimized such that multiplepairs can be grown pseudomorphically to form the hydrogen diffusionbarrier. The thickness of each layer is between about 0.5 nm and 30 nm,with the maximum layer thickness less than or equal to the criticalthickness of the layer, which is determined by the level of strainwithin the material layer. A layer thickness exceeding the criticalthickness relieves the strain energy via dislocations, whereas layerswith thicknesses up to the critical thickness are grownpseudomorphically.

The SLS can comprise at least two different layer types having differentcompositions, strains, and thicknesses. For example, the SLS cancomprise three different layer types or four different layer types. Thetotal thickness of the SLS can be between 50 nm and 6 μm. For example,the total thickness can be between 100 nm and 5 μm, or between 200 nmand 4 μm, or between 500 nm and 3 μm.

Examples of layers that can be used (with strain indicated with respectto growth of layers on a GaAs or Ge substrate) include AlP (tensilestrain), GaP (tensile strain), InAlP (which can be designed to be intensile strain or in compressive strain), InGaP (which can be designedto be in tensile strain or in compressive strain) and InGaAsP (which canalso be designed to be in tensile strain or in compressive strain). TheSLS is optically transparent to wavelengths of light absorbed or emittedby the dilute nitride active layer. Examples of nitrogen-containinglayers that can be used include GaAsN (tensile strain) and InGaAsN(compressive strain), where the bandgap of the nitrogen-containing layeris larger than the dilute nitride active layer, such as a dilute nitridebase layer of a photovoltaic subcell, or a dilute nitride quantum well.Combinations of these materials can also be used. As hydrogen starts todiffuse through the layer, strain within the layers can locally affectthe potential. Additionally, changes in the separation between adjacentatoms within the superlattice can increase the activation energy forhydrogen diffusion, thereby reducing the hydrogen diffusion through thesuperlattice. High bond strength within the superlattice such asbetween, for example, Al—P or Al—Sb can also result in reduceddiffusion. The strain of a layer within the SLS can be between +/−3.5%or between +/−3% or between +/−2% or between +/−1%, with respect to thelattice constant of the device and substrate. For example, a SLS grownon GaAs may comprise In_(x)Al_(1-x)P and In_(y)Al_(1-y)P, with x=0.12and y=0.84 for layers with tensile and compressive strain of about 2.5%,respectively, or with x=0.34 and y=0.62 for layers with tensile andcompressive strain of about 1%, respectively.

For example, a SLS grown on Ge may comprise In_(x)Ga_(1-x)P andIn_(y)Ga_(1-y)P, with x=0.36 and y=0.64 for layers with tensile andcompressive strain of about 1%, or with x=0.3 and y=0.71 for layers withtensile and compressive strain of about 1.5%.

Layer compositions of alloys, including AlP, GaP, InAlP, InGaP, InGaAsP,GaAsN, InGaAsN, GaNAsSb, InGaNAsSb, InAlPSb, InGaPSb, AlInGaAsSb andAlInGaPSb may also be chosen that are either in tensile strain orcompressive strain with respect to substrates such as Si, Ge, GaAs, InPand GaSb.

The higher the strain value, the lower the critical thickness. Strainbalancing between the adjacent layers allows superlattice structures tobe growth with a thickness greater than the critical thickness of asingle layer, without introducing defects into the material. A SLS canbe used to form reflectors such as DBRs.

FIG. 8 shows a simplified cross-section of a photonic device accordingto an embodiment of the invention. The photonic device comprises asubstrate 801 such as GaAs, Ge, or a buffered substrate having a latticeconstant approximately equal to the lattice constant for GaAs. A firstcladding region or a first reflector such as a DBR 803 can be grownusing an MOCVD process 813. At least a portion of the first hydrogendiffusion barrier region 804 overlying DBR/cladding region 803 andunderlying a dilute nitride active layer 805 is grown using an MBEprocess 815. The hydrogen diffusion barrier region 804 is shown as asingle layer for simplicity. However, it will be understood thathydrogen diffusion barrier region 804 may comprise more than one layer,with differing layer compositions, thicknesses, doping levels andstrain, as described herein. In some examples, hydrogen diffusionbarrier region 804 can also be a waveguiding layer and/or a claddingregion, or part of a DBR. MBE process 815 is also used to grow a dilutenitride active layer 805 and at least a portion of the overlyinghydrogen diffusion barrier region 807. The remaining overlying layers(second cladding region 809, top contact layer 811) and a portion of theH-diffusion barrier region 807 (as required) can be grown using an MOCVDprocess (817), allowing the MBE/MOCVD growth interface to be buried inthe hydrogen diffusion barrier region 807. The overlying layer 809 canalso be a second reflector, such as a DBR. The dilute nitride activelayer 805 is shown as a single layer for simplicity. However, it will beunderstood that dilute nitride active layer 805 may comprise more thanone layer, with differing layer compositions, thicknesses, doping levelsand strain, as described herein. The hydrogen diffusion barrier region807 is shown as a single layer for simplicity. However, it will beunderstood that hydrogen diffusion barrier region 807 may comprise morethan one layer, with differing layer compositions, thicknesses, dopinglevels and strain, as described herein.

In some embodiments, dilute nitride active region 805 can include adilute nitride material. The dilute nitride material can be, forexample, containing Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z), where x, y andz can be 0≤x≤0.4, 0<y≤0.07 and 0<z≤0.2, respectively. In someembodiments, x, y and z can be 0.01≤x≤0.4, 0.02≤y≤0.07 and 0.001≤z≤0.04,respectively, and the thickness of the dilute nitride active region canbe within a range, for example, from 0.2 μm to 10 μm or from 1 μm to 4μm. In some embodiments, dilute nitride active region 805 can includequantum well structures to form a dilute nitride active layer. Forexample, a dilute nitride-based active region can include GaInNAs/GaAsor GaInNAsSb/GaAsN multiple quantum wells (MQWs). Examples of dilutenitride-based active regions, including compositions and thicknesses,are described, for example, in U.S. Pat. Nos. 6,798,809, and 7,645,626,each of which is incorporated by reference in its entirety. The dilutenitride material for a quantum well can be, for example,Ga_(1-a)In_(a)NbAs_(≤)Sb_(≤), where a, b, and c can be 0≤a≤0.45,0<b≤0.07 and 0≤c≤0.4, respectively. Several quantum wells may be used,separated by GaAs or GaAsN barriers that provide electron and holeconfinement for the quantum wells. Devices according to FIG. 4 caninclude VCSELs, resonant cavity enhanced photodetectors (RCEPDs),edge-emitting lasers (EELs), light emitting diodes (LEDs),photodetectors, avalanche photodetectors and modulators.

Referring to FIG. 8, hydrogen diffusion barrier region 807 can also be areflector structure and can form part of an overlying DBR. Hydrogendiffusion barrier region 807 can comprise, for example, semiconductormaterials of Groups III and V of the periodic table such as, forexample, AlAs, AlGaAs, GaAs, InAs, InGaAs, AlInAs, InGaP, AlInGaP,InGaP, GaP, InP, AlP, AlInP, or AlInGaAs. An example of a reflector is aDBR. A DBR may comprise, for example, a plurality of alternatingAlGaAs/GaAs layers, with thicknesses selected to provide a desiredreflectivity over a given wavelength range and incident angle. Otherreflector designs may also be used. A first portion of the DBR can beformed using an MBE process 415 (to act as the hydrogen diffusionbarrier region) and a second adjacent overlying portion of the DBR canbe formed using an MOCVD process 817. In other examples, hydrogendiffusion barrier region 807 can also be a waveguiding layer and/or acladding region.

FIG. 9 shows a cross-section of a vertical-cavity surface-emitting laser(VCSEL) 900 according to an embodiment of the present disclosure. VCSEL900 is shown to include a substrate 902, a first reflector layeredstructure 904 overlying the substrate 902; a first spacer region 906overlying the first reflector region; a dilute nitride active layer 908overlying the first spacer region 906; a second spacer region 910overlying the dilute nitride active layer 908; a confinement layer 912overlying second spacer region 910; a contacting layer 914 overlyingconfinement layer 912 (having a first portion 911 and a second portion913); and a second reflector region 916 overlying contacting layer 914.The spacer region 906, dilute nitride active layer 908, and spacerregion 910 define a cavity, which has an associated cavity resonancewavelength. The substrate 902 is made from a semiconductor materialpossessing a corresponding lattice constant. The substrate 902 caninclude gallium arsenide (GaAs), or indium phosphide (InP), but othersemiconductor substrates such as gallium antimonide (GaSb), germanium(Ge), silicon (Si) or an epitaxially grown material (such as a ternaryor quaternary semiconductor), or a buffered or composite substrate canalso be used. The lattice constant of the substrate 902 is judiciouslychosen to minimize defects in materials subsequently grown thereon. Thereflector (or mirror) 904 is typically a semiconductor DBR with alattice matched to that of the substrate 902. A DBR is a periodicstructure formed from alternating materials with different refractiveindices that can be used to achieve high reflection within a range offrequencies or wavelengths. The thicknesses of the layers are chosen tobe an integer multiple of the quarter wavelength, based on a desireddesign wavelength λ₀. That is, the thickness of a layer is chosen to bean integer multiple of λ₀/4n, where n is the refractive index of thematerial at wavelength λ₀. A DBR can include, for example semiconductormaterials of Groups III and V of the periodic table such as, forexample, AlAs, AlGaAs, GaAs, InAs, GaInAs, AlInAs, InGaP, AlInGaP,InGaP, InGaAsP, GaP, InP, AlP, AlInP, or AlInGaAs. When formed on a GaAssubstrate, the DBR is formed using two different compositions forAlGaAs. Mirror 904 can also be doped with an n-type dopant or a p-typedopant to facilitate current conduction through the device structure.Mirror layer 904 may be grown using MBE or MOCVD. A spacer region 906,such as AlGaAs or AlGaInP may be formed overlying the first mirror 904.Dilute nitride active layer 908 is formed overlying spacer layer 906 andincludes a material capable of emitting a substantial amount of light ata desired wavelength of operation. It will be understood that dilutenitride active layer 908 can include various light emitting structures,such as quantum dots, quantum wells, or the like, which substantiallyimprove a light emitting efficiency of VCSEL 900. For a GaAs substrate,the dilute nitride active layer 908 can include a material that can emitlight between a wavelength of about 0.62 μm and 1.6 μm. Dilute nitrideactive layers may be used to emit light between wavelengths of about 1.1μm and 1.6 μm. Active region 908 can include more than one materiallayer but is illustrated as including a single layer in this embodimentfor simplicity and ease of discussion. For example, dilute nitride-basedactive regions can include GaInNAs/GaAs or GaInNAsSb/GaAsN multiplequantum wells (MQWs). Examples of active regions are described in U.S.Pat. Nos. 6,798,809 and 7,645,626, each of which is incorporated byreference in its entirety. A spacer region 910, such as AlGaAs orAlGaInP may be formed overlying dilute nitride active region 908. Aconfinement layer 912 maybe formed overlying spacer region 910. In orderto have efficient VCSEL operation, a method of confining the currentlaterally and/or confining the optical field laterally (providingwaveguiding) is required, thus it is necessary to form a confinementregion within VCSEL 900. In the example shown, confinement layer 912 isformed within VCSEL 900 and has a first portion (confining region 911)and a second portion (aperture 913). Portions 911 and 913 can havedifferent material properties to provide waveguiding and/or to define aregion for current injection such that lasing occurs in an apertureregion 913 within confinement layer 912.

Methods of forming confining regions include, for example, oxidation,ion implantation, semiconductor etching, semiconductor regrowth,deposition of other materials and combinations thereof. In the exampleshown, confining region 911 is formed using oxidation to produce ahighly resistive region with a low refractive index, while defining thelow resistivity aperture 913 through which current can flow. Deviceswith oxide-confined apertures can have very low threshold currents.Aperture 913 is typically circular, so as to form a circular currentinjection region and an associated output beam from VCSEL 900, althoughother shaped apertures such as squares, or rectangles or diamonds mayalso be used. Aperture 913 has a first dimension 926, which in the caseof a circular aperture is the diameter. Contact layer 914 may be formedoverlying confining layer 912. Contact layer 914 can include more thanone material layer but is illustrated as including a single layer.Contact layer 914 is doped with a p-type dopant or an n-type dopant, thedoping type being opposite to the doping type of first mirror 904 inorder to form a p-n junction and to facilitate current conductionthrough the device structure. The doping for contact layer 914 may be aconstant doping level throughout the layer or may comprise regions ofhigh doping such as a delta-doped spike, and low doping to facilitatecurrent spreading and current injection into underlying aperture region913. Examples of p-type dopants for contact layer 914 include Be, C andZn. Examples of n-type dopants for contact layer 914 include Si, Se andTe. Layers 906, 908, 910, 912 and at least a portion of layer 914 may begrown using MBE. Contact layer 914 is described further below.

A second reflector (or mirror) 916 may be formed overlying contact layer914. Second mirror 916 is typically a DBR and is similar in design tofirst mirror 904. When formed on a GaAs substrate, the DBR is formedusing two different compositions for AlGaAs. Second reflector 916 may begrown using MOCVD. VCSEL 900 is completed by a first metal contact 918formed on substrate 902 and a second metal contact 920 formed on contactlayer 914. In the example shown, light emission occurs through the topsurface of reflector 916, as shown. In other examples, light can beemitted through the bottom surface of substrate 902 through an apertureformed in bottom contact 918. p In order to form the device, and inparticular, the confinement and aperture regions, a first mesa structure922 is etched using standard semiconductor etch methods, in order toexpose a higher aluminum-containing layer or layers for oxidation, whichcan be achieved using known methods. For devices formed on a GaAssubstrate, the layer or layers for oxidation typically includeAl_(y)Ga_(1-y)As, where y is greater than 0.9. The oxidation processforms confinement region 911 that has (a) a low refractive index and (b)high resistivity, when compared to the unoxidized aperture region 913,and therefore provides both optical and electrical confinement.

In order to form an electrical contact to contact layer 914, a secondmesa structure 924 is etched using standard semiconductor etch methods,exposing the top surface of contact layer 914, allowing metal contact920 to be formed on contact layer 914 using standard techniques. In thisembodiment, contact layer 914 may also function as a hydrogen diffusionbarrier. Contact layer overlies dilute nitride active layer 908 and isadjacent to confining layer 912. Layers 908, 910, 912 and 914 are allgrown using MBE.

In some embodiments, doping is used within the contact layer, thehydrogen from subsequent MOCVD growth of overlying layers partiallypassivating the dopants, preventing hydrogen diffusion into the dilutenitride active layer 908, while maintaining an acceptable number ofdopants for the layer to function as a contacting layer. In someembodiments the doping for contact layer 914 is carbon. In someembodiments, the doping level for contact layer 914 is carbon. In someembodiments, the doping level lies between 10¹⁷ cm⁻³ and 2×10²⁰ cm⁻³ orbetween 5×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³. The thickness for contact layer 914can be between 25 nm and 1 μm. Contact layer 914 may also be grown as asuperlattice, using for example sublayers with different doping levelsbetween about 10¹⁷ cm⁻³ and 2×10²⁰ cm⁻³, with each sublayer having athickness of at least 1 nm, the plurality of sublayers providing athickness for contact layer 914 between 25 nm and 1 μm.

In some embodiments, contact layer 914 includes at least onenitrogen-containing layer that also acts as a hydrogen diffusion barrierregion. Nitrogen containing layers that may be lattice matched orpseudomorphically strained with respect to a GaAs substrate includeGaAsN, AlGaAsN, GaInAsN, AlNSb, GaNSb, GaInNAsSb, GaNAsSb, GaInNAsBi,GaNAsBi, GaNBi and AlNBi. The bandgap for the nitrogen-containingcontact layer is larger than the bandgap of the dilute nitride activelayer 908 such that it does not absorb light emitted by dilute nitrideactive layer 908. In some embodiments, the dilute nitride material forcontact layer can be Ga_(1-p)In_(p)N_(q)As_(1-q-r)Sb_(r), where p, q andr can be 0≤p<0.4, 0<q<0.07 and 0<r<0.2, respectively. Contact layer 914may also be grown as a SLS, using, for example, sublayers of GaAsN(tensile strain) and InGaAsN (compressive strain), with each sublayerhaving a thickness of at least 1 nm, the plurality of sublayersproviding a thickness for contact layer 914 between 25 nm and 1 μm. Thedoping of each sublayer may also be different to facilitate currentspreading, facilitate electrical contacting and provide a hydrogendiffusion barrier.

While this example shows a hydrogen diffusion barrier region implementedusing a contact layer within the device, the hydrogen diffusion barrierregion may also be implemented in other layers, such as the DBR layersthat form part of second reflector region 916 overlying the contactlayer 914. To function as a hydrogen diffusion barrier, such layers aregrown using MBE, while the remaining overlying DBR layers are grownusing MOCVD.

In some embodiments, the substrate on which the semiconductor layers aregrown can have a miscut. For example, the substrate can be a GaAs or aGe substrate oriented (100) with a miscut between 2 and 10 degreestowards the nearest (110) direction.

In some embodiments, the above-described parameters of layer thickness,composition, doping, strain and substrate miscut can be used incombination to form a hydrogen diffusion barrier region, with at leasttwo of the parameters of layer thickness, composition, doping, strainand substrate miscut selected to provide a hydrogen diffusion barrier.

Changes in performance of the dilute nitride is layers believed to becaused by hydrogen diffusion from the MOCVD growth environment into thedilute nitride material. The presence of nitrogen in the dilute nitridesemiconductor can introduce strong localized potentials due to the largeelectronegativity of N compared to As and Sb, which can attracthydrogen. Hydrogen diffusion from MOCVD growth is also known to causeeffects such as dopant passivation-compensation, introduction ofisolated donors, and may cause other defects such as complex defects ofnitrogen and hydrogen. These effects can change the doping profile ofthe dilute nitride material, resulting in degradation of the electricaland optical performance of a sub-cell. In any embodiment of amultijunction solar cell or a photonic device such as a VCSEL that has alayer of dilute nitride material, a hydrogen getter material (ordiffusion barrier material) caps the dilute nitride before the epiwaferis removed from the low-hydrogen environment of the MBE system. Once inthe MOCVD system, the hydrogen getter preserves the quality of theunderlying dilute nitride by absorbing hydrogen gas on its surface orwithin the layer, thus preventing diffusion of hydrogen into theunderlying dilute nitride active layer. A structure comprising ahydrogen diffusion barrier region overlying a dilute nitride activelayer can reduce the dopant passivation-compensation in the dilutenitride active layer, for example, to less than 1×10¹⁶ cm⁻³, less than1×10¹⁵ cm⁻³, or less than the background doping level of the dilutenitride active layer. A structure comprising a hydrogen diffusionbarrier region overlying a dilute nitride active layer can reduce theintroduction of isolated donors and other defects such as complexdefects of nitrogen and hydrogen into the dilute nitride active layer,for example, to less than 1×10¹⁶ cm⁻³, less than 1×10¹⁵ cm⁻³, or lessthan the background doping level of the dilute nitride active layer.Background doping concentrations, also referred to as the dopantpassivation level, for dilute nitride active layers can be less than5×10¹⁶ cm⁻³ or less than 1×10¹⁶ cm⁻³ or less than 1×10¹⁵ cm⁻³.Background doping levels and doping passivation levels can be measuredusing known measurement techniques such as electrochemicalcapacitance-voltage (ECV) profiling. Defect densities can be measuredusing known measurement techniques including deep-level transientspectroscopy (DLTS), and thermally stimulated current and capacitancemeasurements (TSM). A dilute nitride active layer can have a defectdensity, for example, less than 5×10¹⁶ cm⁻³, less than 1×10¹⁶ cm⁻³, orless than 1×10¹⁵ cm⁻³. Hydrogen incorporation levels within the as-grownMOCVD-grown layers may exceed 1×10¹⁶ cm⁻³ or may exceed 5×10¹⁶ cm⁻³ ormay exceed the defect density level of the dilute nitride active layer.

Additionally, hydrogen within an underlying MOCVD-layer may diffuse intothe MBE-grown materials during MBE growth. A hydrogen getter material(or diffusion barrier material) may therefore be grown before the growthof dilute nitride active layers or regions, preserving the quality of anoverlying dilute nitride by absorbing hydrogen gas at its surface orwithin the layer thus preventing diffusion of hydrogen from anunderlying MOCVD-grown layer into the overlying dilute nitride activelayer. Also, hydrogen from underlying and overlying MOCVD layers maydiffuse into the MBE-grown materials during post-processing steps suchas thermal annealing. A hydrogen diffusion barrier may therefore preventthe diffusion of hydrogen into the dilute nitride active layer.

High performance multijunction solar cells provided by the presentdisclosure can be characterized by an open circuit voltage Voc greaterthan 3.0 V, a fill factor greater than 75%, a short circuit currentdensity Jsc greater than 13 mA/cm², an efficiency greater than 25%, anEg/q-Voc greater than 0.5, measured using a 1 sun AM1.5D source at ajunction temperature of 25° C.

To fabricate solar cells provided by the present disclosure, a pluralityof layers is deposited on a substrate in a first materials depositionchamber. The plurality of layers may include etch stop layers, releaselayers (i.e., layers designed to release the semiconductor layers fromthe substrate when a specific process sequence, such as chemicaletching, is applied), contact layers such as lateral conduction layers,buffer layers, or other semiconductor layers. For example, the sequenceof layers deposited can be a buffer layer(s), then a release layer(s),and then a lateral conduction or contact layer(s). Next the substratecan be transferred to a second materials deposition chamber where one ormore junctions are deposited on top of the existing semiconductorlayers. The substrate may then be transferred to either the firstmaterials deposition chamber or to a third materials deposition chamberfor deposition of one or more junctions and then deposition of one ormore contact layers. Tunnel junctions are also formed between thejunctions.

The movement of the substrate and semiconductor layers from onematerials deposition chamber to another is referred to as a transfer.For example, a substrate can be placed in a first materials depositionchamber, and then the buffer layer(s) and the bottom junction(s) can bedeposited. Then the substrate and semiconductor layers can betransferred to a second materials deposition chamber where the remainingjunctions are deposited. The transfer may occur in vacuum, atatmospheric pressure in air or another gaseous environment, or in anyenvironment in between. The transfer may further be between materialsdeposition chambers in one location, which may or may not beinterconnected in some way or may involve transporting the substrate andsemiconductor layers between different locations, which is known astransport. Transport may be done with the substrate and semiconductorlayers sealed under vacuum, surrounded by nitrogen or another gas, orsurrounded by air. Additional semiconductor, insulating or other layersmay be used as surface protection during transfer or transport, andremoved after transfer or transport before further deposition.

A dilute nitride junction can be deposited in a first materialsdeposition chamber, and the (Al)(In)GaP and (Al)(In)GaAs junctions canbe deposited in a second materials deposition chamber, with tunneljunctions formed between the junctions. A transfer occurs in the middleof the growth of one junction, such that the junction has one or morelayers deposited in one materials deposition chamber and one or morelayers deposited in a second materials deposition chamber.

To fabricate solar cells or photonic devices provided by the presentdisclosure, some or all of the layers of the dilute nitride activelayers and the tunnel junctions can be deposited in one materialsdeposition chamber by molecular beam epitaxy (MBE), and the remaininglayers of the solar cell can be deposited by chemical vapor deposition(CVD) in another materials deposition chamber. For example, a substratecan be placed in a first materials deposition chamber and layers thatmay include nucleation layers, buffer layers, emitter and window layers,contact layers and tunnel junctions can be grown on the substrate,followed by one or more dilute nitride junctions. If there is more thanone dilute nitride junction, then a tunnel junction is grown betweenadjacent junctions. One or more tunnel junction layers may be grown, andthen the substrate can be transferred to a second materials depositionchamber where the remaining solar cell layers are grown by chemicalvapor deposition. In certain embodiments, the chemical vapor depositionsystem is a MOCVD system. In a related embodiment, a substrate is placedin a first materials deposition chamber and layers that may includenucleation layers, buffer layers, emitter and window layers, contactlayers and a tunnel junction are grown on the substrate by chemicalvapor deposition. Subsequently, the top junctions, two or more, aregrown on the existing semiconductor layers, with tunnel junctions grownbetween the junctions. Part of the topmost dilute nitride junction, suchas the window layer, may then be grown. The substrate is thentransferred to a second materials deposition chamber where the remainingsemiconductor layers of the topmost dilute nitride junction may bedeposited, followed by up to three more dilute nitride junctions, withtunnel junctions between them.

A solar cell can be subjected to one or more thermal annealingtreatments after growth. For example, a thermal annealing treatmentincludes the application of a temperature of 400° C. to 1,000° C. forbetween 10 microseconds and 10 hours. Thermal annealing may be performedin an atmosphere that includes air, nitrogen, arsenic, arsine,phosphorus, phosphine, hydrogen, forming gas, oxygen, helium, or anycombination of the preceding materials. In certain embodiments, a stackof junctions and associated tunnel junctions may be annealed prior tofabrication of additional junctions.

Methods provided by the present disclosure include methods of formingthe semiconductor device of claim 1, comprising: depositing the leastone hydrogen diffusion barrier region overlying the dilute nitrideactive layer; and depositing at least one semiconductor layer overlyingthe at least one hydrogen diffusion barrier region, wherein the dilutenitride active layer and the at least one hydrogen diffusion barrierregion are deposited using molecular beam epitaxy (MBE) and the at leastone semiconductor layer is deposited using metal-organic chemical vapordeposition (MOCVD).

Methods provided by the present disclosure include methods for forming asemiconductor device comprising: depositing at least one hydrogendiffusion barrier region overlying a dilute nitride active layer; anddepositing at least one semiconductor layer overlying the at least onehydrogen diffusion barrier region, wherein the dilute nitride activelayer and the at least one hydrogen diffusion barrier region aredeposited using molecular beam epitaxy (MBE) and the at least onesemiconductor layer is deposited using metal-organic chemical vapordeposition (MOCVD), and wherein the dilute nitride active layer has abackground doping concentration less than 10¹⁶ cm⁻³.

While embodiments described herein are for photovoltaic cells comprisinga dilute nitride active layer, the structures and methods described canalso be used in photonic devices including solar cells, photodetectors,optical modulators and lasers; and electronic devices such asheterojunction bipolar transistors (HBT), a high-electron mobilitytransistors (HEMT), a pseudomorphic high-electron mobility transistors(PHEMT), and metal-semiconductor field-effect transistors (MESFET).

ASPECTS OF THE INVENTION

Aspect 1. A semiconductor device comprising a hydrogen diffusion barrierregion overlying a dilute nitride active layer.

Aspect 2. The semiconductor device of aspect 1, wherein the hydrogendiffusion barrier region comprises a pseudomorphically strained layer.

Aspect 3. The semiconductor device of aspect 1, wherein the hydrogendiffusion barrier region comprises a doped semiconductor layer.

Aspect 4. The semiconductor device of aspects 1 to 3, wherein thehydrogen diffusion barrier region comprises AlGaAs, or AlGaAsSb,AlGaAsBi, AlInP, AlInGaP, AlInGaPSb, AlInGaPBi, AlInGaAs, AlInGaAsSb,AlInGaAsBi, AN, AlNSb, and AlNBi.

Aspect 5. The semiconductor device of any one of aspects 1 to 4, whereinthe hydrogen diffusion barrier region comprises AlGaAs.

Aspect 6. The semiconductor device of any one of aspects 1 to 3, whereinthe hydrogen diffusion barrier comprises a nitrogen-containing layer.

Aspect 7. The semiconductor device of any one of aspects 1 to 3, whereinthe hydrogen diffusion barrier region has a thickness within a rangefrom 50 nm to 6 μm.

Aspect 8. The semiconductor device of any one of aspects 1 to 7, whereinthe dilute nitride active layer comprises GaNAs, GaInNAs, GaInNAsSb,GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi or GaNAsSbBi.

Aspect 9. The semiconductor device of any one of aspects 1 to 8, whereinthe dilute nitride active layer comprises GaAsN, AlGaAsN, GaInAsN, GaN,AN, AlNSb, GaNSb, GaInNAsSb, GaNBi or AlNBi.

Aspect 10. The semiconductor device of any one of aspects 1 to 9,wherein the hydrogen diffusion barrier region and the dilute nitrideactive layer are grown by molecular beam epitaxy.

Aspect 11. The semiconductor device of any one of aspects 1 to 10,wherein, the semiconductor device comprises a plurality of semiconductorlayers; and each of the plurality of semiconductor layers issubstantially lattice matched or pseudomorphically strained to each ofthe other semiconductor layers.

Aspect 12. The semiconductor device of any one of aspects 1 to 11,wherein the semiconductor device comprises a multijunction solar cell.

Aspect 13. The semiconductor device of aspect 12, wherein themultijunction solar cell comprises: a substrate underlying the dilutenitride active layer; and one or more junctions overlying the hydrogendiffusion barrier region.

Aspect 14. The semiconductor device of aspect 13, wherein the substratecomprises GaAs, InP, GaSb, (Sn,Si)Ge, or silicon.

Aspect 15. The semiconductor device of any one of aspects 12 to 14,wherein each of the one or more junctions independently comprisesAlInGaP or (Al)(In)GaAs.

Aspect 16. The semiconductor device of aspect 13, wherein, the substratecomprises (Sn,Si)Ge; the dilute nitride active layer comprisesGaInNAsSb; and each of the one or more junctions independently comprisesAlInGaP or InAlGaAs.

Aspect 17. The semiconductor device of any one of aspects 12 to 16,wherein each of the hydrogen diffusion barrier region, the dilutenitride active layer, the substrate, and the one or more secondjunctions, is substantially lattice matched or pseudomorphicallystrained to each other.

Aspect 18. The semiconductor device of any one of aspects 14 to 17,wherein, each of the dilute nitride active layer and the hydrogendiffusion barrier region is grown by molecular beam epitaxy; and each ofthe substrate and the one or more junctions is grown by metal-organicchemical vapor deposition.

Aspect 19. The semiconductor device of any one of aspects 12 to 18,wherein the multijunction solar cell is a four-junction multijunctionsolar cell, and is characterized by an open circuit voltage Voc greaterthan 3.0 V, a fill factor greater than 75%, a short circuit currentdensity Jsc greater than 13 mA/cm², an efficiency greater than 25%, anEg/q-Voc greater than 0.5, measured using a 1 sun AM1.5D source at ajunction temperature of 25° C.

Aspect 20. A method of fabricating a semiconductor device comprising adilute nitride active layer, comprising providing a substrate; growing adilute nitride active layer overlying the substrate using molecular beamepitaxy; growing a hydrogen diffusion barrier region overlying thedilute nitride active layer using molecular beam epitaxy; applying afirst thermal treatment to the substrate, the dilute nitride activelayer, and the hydrogen diffusion barrier region; growing one or moresemiconductor layers overlying the annealed hydrogen diffusion barrierregion using metal-organic chemical vapor deposition; and applying asecond thermal treatment to the substrate, the dilute nitride activelayer, the hydrogen diffusion barrier region, and the one or moresemiconductor layers.

Aspect 21. The method of aspect 20, wherein the first thermal treatmentcomprises rapid thermal annealing.

Aspect 22. The method of any one of aspects 20 to 21, wherein the rapidthermal annealing comprises applying a temperature within a range from600° C. to 900° C. for a duration from 5 seconds to 3 hours.

Aspect 23. The method of any one of aspects 20 to 22, wherein the secondthermal treatment comprises applying a temperature within a range from400° C. to 1,000° C. for between 10 microseconds and 10 hours.

Aspect 24. The method of any one of aspects 20 to 23, wherein the dilutenitride active layer comprises GaNAs, GaInNAs, GaInNAsSb, GaInNAsBi,GaInNAsSbBi, GaNAsSb, GaNAsBi or GaNAsSbBi.

Aspect 25. The method of any one of aspects 20 to 24, wherein the dilutenitride active layer comprises GaInNAsSb.

Aspect 26. The method of any one of aspects 20 to 25, wherein each ofthe substrate, the dilute nitride active layer, the hydrogen diffusionbarrier region, and the one or more semiconductor layers issubstantially lattice matched to each of the other layers.

Aspect 27. The method of any one of aspects 20 to 26, wherein thesemiconductor device comprises a multijunction solar cell.

Aspect 28. The method of any one of aspects 20 to 27, wherein, thesubstrate comprises GaAs, InP, GaSb, (Sn,Si)Ge, or silicon; the dilutenitride active layer comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi,GaNAsSb, GaNAsBi or GaNAsSbBi; and each of the one or more semiconductorlayers independently comprises AlInGaP or (Al)(In)GaAs.

Aspect 29. The method of any one of aspects 20 to 28, wherein thesubstrate comprises GaAs, InP, GaSb, (Sn,Si)Ge, or silicon.

Aspect 30. The method of any one of aspects 20 to 29, wherein each ofthe one or more semiconductor layers independently comprises AlInGaP or(Al)(In)GaAs.

Aspect 31. The method of any one of aspects 20 to 30, wherein, thesubstrate comprises Ge; the dilute nitride active layer comprisesGaInNAsSb; and each of the or more semiconductor layers independentlycomprises AlInGaP or InAlGaAs.

Aspect 32. The semiconductor device of any one of aspects 1 to 11,wherein the semiconductor device comprises a semiconductor laser.

Aspect 1A. A semiconductor device comprising a dilute nitride activelayer, wherein the dilute nitride active layer comprises: a dilutenitride material selected from GaInNAsSb, GaInNAsBi, GaNAs, GaInNAs,GaInNAsSbBi, GaNAsSb, GaNAsBi, and GaNAsSbBi; a background dopingconcentration less than 5×10¹⁶ cm⁻³; and a hydrogen-induced defectdensity less than the background doping density; a hydrogen diffusionbarrier region overlying the dilute nitride active layer, wherein thehydrogen diffusion barrier region comprises a doped semiconductor layer,a dilute nitride semiconductor layer, a strained semiconductor layer, ora combination of any of the foregoing; and one or more semiconductorlayers overlying the hydrogen diffusion barrier region.

Aspect 2A. The semiconductor device of aspect 1A, wherein the hydrogendiffusion barrier region is adjacent the dilute nitride active layer,without any intervening semiconductor layers.

Aspect 3A. The semiconductor device of any one of aspects 1 to 2,wherein the hydrogen diffusion barrier region has a thickness within arange from 25 nm to 6 μm.

Aspect 4A. The semiconductor device of any one of aspects 1A to 2A,wherein the hydrogen diffusion barrier region comprises a dopedsemiconductor layer.

Aspect 5A. The semiconductor device of aspect 4A, wherein the dopedsemiconductor layer comprises a dopant selected from C, Be, Zn, Si, Se,Te, and a combination of any of the foregoing.

Aspect 6A. The semiconductor device of any one of aspects 4A to 5A,wherein the doped semiconductor layer comprises a doping level between1×10¹⁷ cm⁻³ and 2×10²⁰ cm⁻³.

Aspect 7A. The semiconductor device of any one of aspects 1 to 6,wherein the hydrogen diffusion barrier region comprises a dilute nitridesemiconductor layer.

Aspect 8A. The semiconductor device of aspect 7A, wherein, the dilutenitride active layer comprises a first bandgap; the dilute nitridesemiconductor layer comprises a second bandgap; and the second bandgapis larger than the first bandgap.

Aspect 9A. The semiconductor device of any one of aspects 7A to 8A,wherein the dilute nitride semiconductor layer comprises GaAsN, AlGaAsN,GaInAsN, GaN, AN, AlNSb, GaNSb, GaInNAsSb, GaNBi or AlNBi.

Aspect 10A. The semiconductor device of any one of aspects 1A to 9A,wherein the hydrogen diffusion barrier region comprises a strainedsemiconductor layer.

Aspect 11A. The semiconductor device of aspect 10A, wherein, thesemiconductor device further comprises a substrate underlying the dilutenitride active layer; and the strained semiconductor layer has strainwithin a range from +/−3.5% with respect to the substrate.

Aspect 12A. The semiconductor device of any one of aspects 10A to 11A,wherein the strained semiconductor layer is a strained superlatticestructure (SLS).

Aspect 13A. The semiconductor device of any one of aspects 1A to 12A,wherein the hydrogen diffusion barrier region does not contain aluminum.

Aspect 14A. The semiconductor device of any one of aspects 1A to 13A,wherein each of the dilute nitride active layer and the hydrogendiffusion barrier region is grown by molecular beam epitaxy.

Aspect 15A. The semiconductor device of any one of aspects 1A to 14A,wherein the semiconductor layer adjacent the hydrogen diffusion barrierregion is grown by MOCVD.

Aspect 16A. The semiconductor device of any one of aspects 1A to 15A,further comprising a hydrogen diffusion barrier region underlying thedilute nitride active layer.

Aspect 17A. The semiconductor device of any one of aspects 1A to 16A,wherein the semiconductor device comprises a solar cell comprising oneor more junctions.

Aspect 18A. The semiconductor device of aspect 17A, wherein, the solarcell comprises a dilute nitride junction comprising a dilute nitridebase layer; and the dilute nitride base layer comprises the dilutenitride active layer.

Aspect 19A. The semiconductor device of any one of aspects 17A to 18A,further comprising a substrate underlying the dilute nitride junction.

Aspect 20A. The semiconductor device of aspect 19A, wherein thesubstrate comprises GaAs, InP, GaSb, germanium, or silicon.

Aspect 21A. The semiconductor device of any one of aspects 19A to 20A,wherein the substrate comprises (Sn,Si)Ge; and the dilute nitride activelayer comprises GaInNAsSb.

Aspect 22A. The semiconductor device of any one of aspects 19A to 21A,wherein each of the hydrogen diffusion barrier region, the substrate,and the one or more junctions, is substantially lattice matched to eachother.

Aspect 23A. The semiconductor device of any one of aspects 19A to 22A,wherein, each of the dilute nitride active layer and the hydrogendiffusion barrier region is grown by molecular beam epitaxy; and each ofthe substrate and the semiconductor layer adjacent the hydrogendiffusion barrier region is grown by metal-organic chemical vapordeposition.

Aspect 24A. The semiconductor device of any one of aspects 17A to 23A,wherein, the solar cell comprises a plurality of semiconductor layers;and each of the plurality of semiconductor layers is substantiallylattice matched to each of the other semiconductor layers.

Aspect 25A. The semiconductor device of any one of aspects 17A to 24A,wherein the solar cell comprises a multijunction solar cell.

Aspect 26A. The semiconductor device of aspect 25A, wherein themultijunction solar cell is a four-junction multijunction solar cell,and is characterized by an open circuit voltage Voc greater than 3.0 V,a fill factor greater than 75%, a short circuit current density Jscgreater than 13 mA/cm², an efficiency greater than 25%, an Eg/q-Vocgreater than 0.5, measured using a 1 sun AM1.5D source at a junctiontemperature of 25° C.

Aspect 27A. The semiconductor device of any one of aspects 19A to 26A,wherein the semiconductor device comprises a solar cell, a verticalcavity surface emitting laser, a resonant cavity enhanced photodetector,an edge-emitting laser, a light emitting diode, a photodetector, anavalanche photodetector, or an optoelectronic modulator.

Aspect 28A. A method of fabricating a semiconductor device comprising adilute nitride active layer, comprising: growing a dilute nitride activelayer overlying a substrate using molecular beam epitaxy; growing ahydrogen diffusion barrier region overlying the dilute nitride activelayer using molecular beam epitaxy; thermally annealing the substrate,the dilute nitride active layer, and the hydrogen diffusion barrierregion; and growing a semiconductor layer adjacent the thermallyannealed hydrogen diffusion barrier region using metal-organic chemicalvapor deposition.

Aspect 29A. The method of aspect 28A, wherein growing a hydrogendiffusion barrier region comprises growing the hydrogen diffusionbarrier region adjacent the dilute nitride active layer using molecularbeam epitaxy.

Aspect 30A. The method of aspect 28A, wherein further comprising: aftergrowing the dilute nitride active layer and before growing the hydrogendiffusion barrier region, growing one or more intervening semiconductorlayers overlying the dilute nitride active layer using molecular beamepitaxy; and growing the hydrogen diffusion barrier region comprisesgrowing the hydrogen diffusion barrier region adjacent an uppermostintervening semiconductor layer.

Aspect 31A. The method of any one of aspects 28A to 30A, whereinthermally annealing the substrate, the dilute nitride active layer, andthe hydrogen diffusion barrier region comprises rapid thermal annealing.

Aspect 32A. The method of aspect 3A1, wherein the rapid thermalannealing comprises applying a temperature within a range from 600° C.to 900° C. for a duration from 5 seconds to 3 hours.

Aspect 33A. The method of any one of aspects 28A to 32A, whereincomprising, after growing the semiconductor layer, thermally annealingthe substrate, the dilute nitride active layer, the hydrogen diffusionbarrier region, and the semiconductor layer.

Aspect 34A. The method of aspect 33A, wherein thermally annealing thesubstrate, the dilute nitride active layer, the hydrogen diffusionbarrier region, and the semiconductor layer comprises applying atemperature within a range from 400° C. to 1,000° C. for between 10microseconds and 10 hours.

Aspect 35A. The method of any one of aspects 28A to 34A, wherein thedilute nitride active layer comprises GaNAs, GaInNAs, GaInNAsSb,GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi or GaNAsSbBi.

Aspect 36A. The method of any one of aspects 28A to 35A, wherein thedilute nitride active layer comprises GaInNAsSb.

Aspect 37A. The method of any one of aspects 28A to 37A, wherein each ofthe substrate, the dilute nitride active layer, the hydrogen diffusionbarrier region, and the semiconductor layer is substantially latticematched to each of the other layers.

Aspect 38A. The method of any one of aspects 28A to 38A, wherein, thesemiconductor device comprises a multijunction solar cell comprising twoor more junctions; and one of the junctions comprises a dilute nitridejunction comprising the dilute nitride active layer.

Aspect 39A. The method of aspect 38A, wherein, the substrate comprisesGaAs, InP, GaSb, (Sn,Si)Ge, or silicon; and the dilute nitride activelayer comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi orGaNAsSbBi.

Aspect 40A. The method of any one of aspects 38 to 39, wherein themultijunction solar cell comprises a junction comprising an AlInGaP baselayer and/or a junction comprising a (Al)(In)GaAs base layer overlyingthe hydrogen diffusion barrier region.

Aspect 41A. The method of any one of aspects 38A to 40A, wherein, thesubstrate comprises Ge; the dilute nitride active layer comprisesGaInNAsSb; and the multijunction solar cell comprises a junctioncomprising an AlInGaP base layer and/or a junction comprising a(Al)(In)GaAs base layer overlying the hydrogen diffusion barrier region.

Aspect 42A. The method of any one of aspects 28A to 41A, wherein thesemiconductor device comprises a solar cell, a vertical cavity surfaceemitting laser, a resonant cavity enhanced photodetector, anedge-emitting laser, a light emitting diode, a photodetector, anavalanche photodetector or an optoelectronic modulator.

It should be noted that there are alternative ways of implementing theembodiments disclosed herein. Accordingly, the present embodiments areto be considered as illustrative and not restrictive. Furthermore, theclaims are not to be limited to the details given herein and areentitled their full scope and equivalents thereof.

1. A method of fabricating a semiconductor device comprising a dilutenitride active layer, comprising: growing a dilute nitride active layeroverlying a substrate using molecular beam epitaxy, the dilute nitridelayer comprising: a dilute nitride material selected from GaNAs,GaInNAs, GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, andGaNAsSbBi, a background doping concentration less than 5×10¹⁶ cm⁻³, anda hydrogen-induced defect density less than the background dopingdensity; growing a hydrogen diffusion barrier region overlying thedilute nitride active layer using molecular beam epitaxy, wherein thehydrogen diffusion barrier region comprises a doped semiconductor layer,a dilute nitride semiconductor layer, a strained semiconductor layer, ora combination of any of the foregoing; thermally annealing thesubstrate, the dilute nitride active layer, and the hydrogen diffusionbarrier region; and growing a semiconductor layer adjacent the hydrogendiffusion barrier region using metal-organic chemical vapor deposition.2. The method of claim 1, further comprising: after growing the dilutenitride active layer and before growing the hydrogen diffusion barrierregion, growing one or more intervening semiconductor layers overlyingthe dilute nitride active layer using molecular beam epitaxy, whereingrowing the hydrogen diffusion barrier region comprises growing thehydrogen diffusion barrier region adjacent an uppermost interveningsemiconductor layer.
 3. The method of claim 2, wherein growing the oneor more intervening semiconductor layers comprises growing (In)GaAs,(Al)GaAs, or both.
 4. The method of claim 1, wherein thermally annealingthe substrate, the dilute nitride active layer, and the hydrogendiffusion barrier region comprises rapid thermal annealing.
 5. Themethod of claim 1, further comprising: after growing the semiconductorlayer, thermally annealing the substrate, the dilute nitride activelayer, the hydrogen diffusion barrier region, and the semiconductorlayer.
 6. The method of claim 1, wherein growing the hydrogen diffusionbarrier comprises doping the doped semiconductor layer with a dopantselected from C, Be, Zn, Si, Se, Te, and a combination of any of theforegoing.
 7. The method of claim 1, wherein the doped semiconductorlayer comprises a doping level between 1×10¹⁷ cm⁻³ and 2×10²⁰ cm⁻³. 8.The method of claim 1, wherein the dilute nitride semiconductor layercomprises GaAsN, AlGaAsN, GaInAsN, GaN, AN, AlNSb, GaNSb, GaInNAsSb,GaNBi or AlNBi.
 9. The method of claim 1, wherein the dilute nitrideactive material comprises Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z), wherein:0≤x≤0.24, 0.001≤y≤0.07, and 0.001≤z≤0.2; 0.08≤x≤0.24, 0.02≤y≤0.05, and0.001≤z≤0.02; 0.07≤x<0.18, 0.025≤y≤0.04, and 0.001≤z≤0.03; or 0≤x≤0.4,0<y≤0.07, and 0<z≤0.04.
 10. The method of claim 1, wherein the strainedsemiconductor layer is a strained superlattice structure (SLS).
 11. Themethod of claim 1, wherein the hydrogen diffusion barrier regioncomprises an aluminum-containing layer.
 12. The method of claim 1,further comprising: growing a nucleation layer and at least a portion ofa buffer layer using metal-organic chemical vapor deposition.
 13. Themethod of claim 12, further comprising: growing another portion of thebuffer layer using molecular beam epitaxy.
 14. The method of claim 1,further comprising: growing a second semiconductor layer overlying thesemiconductor layer using metal-organic chemical vapor deposition. 15.The method of claim 1, wherein growing the semiconductor layer comprisesgrowing a Distributed Bragg Reflector.
 16. The method of claim 1,further comprising: growing a protective layer overlying the hydrogendiffusion barrier region using molecular beam epitaxy.
 17. The method ofclaim 1, wherein the hydrogen diffusion barrier region comprises AlAs,AlGaAs, GaAs, InAs, InGaAs, AlInAs, InGaP, AlInGaP, InGaP, GaP, InP,AlP, AlInP, or AlInGaAs.
 18. The method of claim 1, further comprising:transferring the semiconductor device after growing the hydrogendiffusion barrier region and before growing the semiconductor layer,wherein the transferring comprises transferring the semiconductor devicein atmospheric pressure.
 19. The method of claim 1, further comprising:transporting the semiconductor device after growing the hydrogendiffusion barrier region and before growing the semiconductor layer,wherein the transporting comprises transporting the semiconductor devicein vacuum.
 20. The method of claim 1, wherein the molecular beam epitaxyis performed in a molecular beam epitaxy system and the metal-organicchemical vapor deposition is performed in a metal-organic chemical vapordeposition system, further comprising: growing a protective layer beforetransferring or transporting the semiconductor device from the molecularbeam epitaxy system to the metal-organic chemical vapor depositionsystem.